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How Jack Flaherty turned the corner (Part One)

A pitch-by-pitch breakdown of the young righty’s sudden success against lefties

Photo by Christian Petersen/Getty Images

(Author’s note: As I mentioned in an earlier post, this piece on Flaherty has been in the works for a little while now. I chose to put it on hold in light of the Matheny firing, so the stats wherein are not updated to include his July 20 outing against the Cubs. I apologize for the inconvenience. On a somewhat related note, this article was dangerously flirting with a word count in excess of 5,000 words had I merged the entirety of this pitch-by-pitch analysis into one post. Instead, I will focus today on Flaherty’s breaking pitches while the second part will be dedicated to the multiple fastball variations that he throws.)

You don’t need me to tell you that Jack Flaherty has opened people’s eyes this year. After excelling at every stop he made in the minor leagues, an admittedly raw 21-year-old encountered the first notable struggles of his professional career when he was dinged for a 6.33 ERA and 5.27 FIP in six MLB games over the tail end of 2017. Nevertheless, his continued success throughout the minors earned him national pedigree as a consensus top 100 prospect. When I compiled aggregate prospect rankings back in May, MLB Pipeline slotted Flaherty as aggressively as its 33rd best prospect in all of baseball.

Round two in the majors has been far more impressive. If his 3.15 ERA–which is tied for 27th among the 158 starters with at least 40 innings this season–doesn’t sell you, Flaherty also boasts the advanced metrics to back it up. SIERA and kwERA both utilize strikeout and walk rates (the former also includes groundball rate in its formula) to better capture the elements of a pitcher’s performance that he has control over. In a study of eight metrics at FanGraphs, Matt Swartz found these two to have the most predictive power. By these stats, Flaherty ranks even higher at 22nd and 20th, respectively, placing second only to Walker Buehler of the Dodgers among 20 qualified rookies.

This dichotomy between 2017 Jack Flaherty and 2018 Jack Flaherty begs the question behind today’s article: in what ways has he improved to make the leap forward in his development? More specifically I’ll be highlighting his ascension against left-handed batters, which should make more sense after comparing his lefty/righty splits from this season to last.

Jack Flaherty: Change in Lefty/Righty Splits

Stat 2017 vs. LHH 2018 vs. LHH Change 2017 vs. RHH 2018 vs. RHH Change
Stat 2017 vs. LHH 2018 vs. LHH Change 2017 vs. RHH 2018 vs. RHH Change
wOBA 0.448 0.265 -0.183 0.266 0.300 0.034
xwOBA 0.412 0.290 -0.122 0.258 0.282 0.024
Opp. OPS 1.096 0.589 -0.507 0.588 0.695 0.107
K% 13.0% 21.8% 8.8% 29.2% 34.6% 5.4%
BB% 10.9% 9.6% -1.3% 10.4% 5.9% -4.5%

For context, a .448 wOBA like Flaherty’s 2017 mark against lefties is higher than Mike Trout’s overall wOBA this season while his .265 wOBA versus lefties in 2018 is on par with...2006 Yadier Molina at the plate (minus the soul-crushing homer that still haunts Mets fans to this day). As for righties, Flaherty has taken a step back by virtue of batters making harder and higher contact. Their average exit velocity against him has ticked up from 83.8 mph to 85.4 mph and the average launch angle has more than doubled from 7° flat to 16.1° this year. However, some of this regression has been offset by striking more righties out and simultaneously yielding fewer walks. In fact, his K-BB% of 28.7% trails only Max Scherzer, Chris Sale, and Stephen Strasburg for starting pitchers logging at least 25 innings facing righties.

Let’s first dive into Flaherty’s pitch breakdown against lefties and how it has evolved since last season.

Jack Flaherty: Changes in Pitch Usage vs. Lefties

Pitch Type 2017 2018 Change
Pitch Type 2017 2018 Change
Slider 8.29% 21.81% 13.52%
Fourseamer 39.90% 41.69% 1.79%
Sinker 15.03% 14.99% -0.04%
Changeup 11.92% 6.82% -5.10%
Curve 24.87% 14.69% -10.18%

The changeup, battered and beaten for a 1.320 wOBA and 0.919 xwOBA (expected wOBA according to exit velocity and launch angle data from Statcast) against lefties in 2017, has become a mere afterthought in Flaherty’s arsenal. The only significant gainer in usage rate is the slider, usurping the curveball as Flaherty’s go-to secondary pitch to compliment the fastball. The former type of breaking ball has been much more effective in 2018 than the 2017 edition of the latter.

Jack Flaherty’s Favorite Secondary Pitch vs. Lefties

Metric 2017 Curveball 2018 Slider Difference
Metric 2017 Curveball 2018 Slider Difference
wOBA 0.350 0.250 -0.100
xwOBA 0.320 0.209 -0.111
K% 25.0% 47.6% 22.6%


Eno Sarris hailed Flaherty’s slider as the best of any starter in the second half last season given its propensity to generate whiffs and groundballs, yet opposing lefty batters–albeit in a small sample size–tagged the pitch for a .408 wOBA and xwOBA in 2017. Its average spin rate appears to have jumped from 2,172 to 2,221 rotations per minute (rpm), but I noticed that one such slider against Denard Span in his MLB debut was listed at just 420 rpm. This is presumably a measurement error on Statcast’s end, as the pitch had the second lowest spin rate of any individual slider thrown in September last year. (Dan Otero had a slider recorded at 413 rpm, with his next lowest spin rate being 1,945 rpm.) When discarding this outlier and recalculating Flaherty’s average slider spin rate, we get a 2,208 rpm much closer in line with his 2018 mark.

That said, virtually identical spin rate numbers don’t mean the slider hasn’t improved. Pitch Info, which supplies much of the data you might find on, says its horizontal and vertical movement has increased by 1.6 and 0.3 inches, respectively. Of particular note is the horizontal movement Flaherty has added to the slider. Looking at pitchers who threw at least 20 innings last year, 60 frames this year, and overall used their slider at least 20% of the time in both seasons, he ranks seventh in terms of horizontal movement added. According to QOP Baseball, that vaults the slider’s horizontal movement from the 34th percentile to the 61st.

Pitchers With the Most Added Horizontal Movement (Sliders)

Name Current Team 2017 2018 Increase in Movement (Inches)
Name Current Team 2017 2018 Increase in Movement (Inches)
Michael Fulmer Tigers 1.2 5.0 3.8
Chris Sale Red Sox -5.6 -8.0 2.4
Luis Severino Yankees 3.7 6.1 2.4
Justin Verlander Astros 0.3 2.2 1.9
Carlos Martinez Cardinals 5.2 6.9 1.7
Masahiro Tanaka Yankees 2.3 4.0 1.7
Jack Flaherty Cardinals 1.6 3.2 1.6
Tyson Ross Padres 1.8 3.1 1.3
Matt Harvey Reds 0.1 1.3 1.2
Corey Kluber Indians 7.8 8.9 1.1

(Side tangent: The Yankees and Cardinals both have two pitchers on this list, and Corey Kluber’s teammate Carlos Carrasco was the next man up, meaning that three teams control the majority of the top 11. Furthermore, New York, Cleveland, and St. Louis collectively rank 30th, 24th, and 20th in fastball usage rate despite having average or better fastball velocity. Yankees pitchers throw the hardest heaters in baseball while the Cardinals aren’t far behind in fourth place. Perhaps this warrants a more thorough examination in a future post, but for now this is simply a trend I felt compelled to bring to your attention. We now return to your regularly scheduled programming.)

One benefit of Flaherty’s slider compared to the curveball he displayed last season is that it “tunnels” more favorably, essentially allowing him to better deceive the hitter as to what pitch is coming next. According to Baseball Prospectus’ pitch tunneling data, the average distance in release point between two pitches for Flaherty was 2.7 inches against lefties in 2017, a below average number that places him around the 40th percentile. That gap in release points has shrunk to just 1.93 inches this season. While that may not sound like much, it’s enough to propel him all the way into the 82nd percentile based on research conducted by former VEB writer Andy Schrag. (I highly recommend you read his pitch tunneling primer that I linked to in the previous sentence.)

To illustrate the effects of release points and how they can help (or hurt) a pitcher in terms of “tipping” his pitches, I queued up video from Flaherty’s major league debut (in which he surrendered five earned runs in four innings) and his June 22 start from this year against the Brewers (when he fanned 13 batters and carried a no-hitter into the seventh inning).

First up is the 2017 outing. I’ll tell you upfront that one pitch in the video below is a curveball; the other is a fastball. Versus lefties, Flaherty’s average fastball release point last year was 3.4 inches further away from the pitching rubber and 1.3 inches lower relative to the curveball. Knowing that the higher arm slot belongs to the curveball, see if you can identify which pitch is which.

Hopefully you were able to discern the two without much trouble. The release point at which the ball is released coming out of the hand can be a crucial piece of information for hitters, who only have a matter of milliseconds to recognize a pitch and decide whether or not they are going to swing.

Now we’ll turn our attention to the game where Flaherty mowed down Milwaukee’s lineup. This time, the video will display a fastball and slider. Their average release points in 2018 are separated by a mere 0.1 and 0.7 inches horizontally and vertically, with the fastball owning the slightly highly arm slot. These two pitches are a bit more difficult to quickly distinguish, much to the chagrin of Eric Thames as you are about to find out.

With the exception of the first pitch fastball, second pitch fastball pairing, the most common 1-2 combo against lefties for Flaherty was fastball-curveball last season and fastball-slider this year. The curveball was his leading secondary pitch with two strikes in 2017 at 27% usage versus left-handed batters, which has slimmed down to 9% this year while the slider’s two-strike usage has ballooned to 35%. The aforementioned Baseball Prospectus pitch tunneling metrics allow us to juxtapose the two put-away pitch sequences. Here is a brief explanation of the three stats in the table to follow:

  • Release Distance: The concept we just discussed with the side-by-side video clips. This is simply a measure of the difference in release points for back-to-back pitches.
  • Pre-Tunnel Max Distance: The perceived distance between two pitches at the “point of no return,” or the final point at which the batter still has time to swing.
  • Plate:PreMax Ratio: This essentially quantifies how much pitches move between the point of no return and when they arrives at home plate, with a higher number indicating more “late break.” Granted, physics doesn’t actually allow for late break, but that is what we are measuring here as it pertains to baseball analysis.

Jack Flaherty: Slider vs. Curveball Pitch Tunneling vs. Lefties

Metric 2017 Fastball-Curveball 2018 Fastball-Slider Difference
Metric 2017 Fastball-Curveball 2018 Fastball-Slider Difference
Release Distance 3.93 2.49 -1.44
Pre-Tunnel Max Distance 1.65 1.53 -0.12
Plate:PreMax Ratio 14.1 14.8 0.7

We already knew the 2018 slider’s release point better mimicked that of the fastball than the 2017 curveball, but this would also suggest the slider looks more similar to the fastball at the “decision point” for batters–leaping from the 21st percentile to what I would approximate to be the 51st in that category–and deviates more drastically in path after the hitter has already made their decision to swing or not to swing. (As an aside, you should keep in mind that we don’t have a long history of interpreting this data and different types of pitches likely have different benchmarks concerning what are considered “good” pitch tunneling numbers.) The fastball-slider pairing also enjoys a very low pre-tunnel max time of 156 milliseconds, meaning the two pitches separate later in their flight paths, thus making it even more difficult to differentiate the two. Hitting a baseball is really hard.

As a matter of fact, Flaherty owns the ninth lowest pre-tunnel max time among all pitchers with at least 40 fastball-sliders combos against lefties this season.

Pitchers With the Lowest Pre-Tunnel Max Time, Fastball-Slider Pairings

Name Release Distance Pre-Tunnel Max Distance Pre-Tunnel Max Time Plate Distance Flight Time Differential Plate:PreMax Ratio
Name Release Distance Pre-Tunnel Max Distance Pre-Tunnel Max Time Plate Distance Flight Time Differential Plate:PreMax Ratio
Julio Teheran 1.51 1.49 0.154 24.64 0.0527 16.5
Ross Stripling 2.80 1.71 0.154 18.77 0.0169 11
Amir Garrett 3.92 1.87 0.155 19.58 0.0577 10.4
Matt Boyd 4.01 1.66 0.155 20.57 0.0496 12.4
Cam Bedrosian 2.76 1.98 0.155 25.55 0.048 12.9
Tyson Ross 1.63 1.52 0.155 21.13 0.0325 13.9
Dan Straily 2.22 1.60 0.155 18.06 0.0318 11.3
Jacob Barnes 2.91 1.67 0.155 22.27 0.021 13.3
Jack Flaherty 2.49 1.53 0.156 22.67 0.0456 14.8
Chad Bettis 1.79 1.37 0.156 15.75 0.0223 11.5

Even when just analyzing this top 10 leaderboard, Flaherty grades out as above average in hiding whether he is throwing a fastball or slider with similar release points, disguising the two as similar early in their flight paths, but also at throwing hitters off with vastly different movement/location (see: plate distance) and speeds (see: flight time differential) after it’s too late for the opponent to change their mind.

Bottom line: this is arguably the most deadly one-two punch that Flaherty possesses.


For as much as it may sound like I was just bashing the curveball while it took a backseat to the slider this year, “Uncle Charlie” too has made sound improvements to enhance Flaherty’s repertoire against lefties.

The story begins with sharpened command as the curveball’s optimal location rating on QOP Baseball has skyrocketed from the 22nd to the 64th percentile. Look at how the 2018 pitch heatmap (the lower of the two) features a darker-colored core concentrated solely in the down-and-away corner.

Baseball Savant
Baseball Savant

The key is preventing the curveball from creeping towards the middle or inner portions of the plate. In 2017, Flaherty allowed a sterling–from a pitcher’s perspective, that is–.173 wOBA to lefties on curveballs located in the outer third of the zone compared to .527 against curveballs on the innermost two thirds.

Like the slider, the curveball has added its fair share of horizontal movement. Enough, as it just so happens, for Flaherty to work his way onto another top 10 list. (Minimum 20 innings pitched in 2017 and 60 in 2018 with at least 5% curveball usage in both seasons.)

Pitchers With the Most Added Horizontal Movement (Curveballs)

Name Current Team 2017 2018 Increase in Movement
Name Current Team 2017 2018 Increase in Movement
Lucas Giolito White Sox 2.6 5.6 3.0
Charlie Morton Astros 8.1 10.4 2.3
Aaron Sanchez Blue Jays 6.4 8.5 2.1
Jack Flaherty Cardinals 6.3 8.0 1.7
Tyler Chatwood Cubs 3.7 5.4 1.7
Chase Anderson Brewers 3.1 4.8 1.7
Jason Hammel Royals 5.7 7.3 1.6
Julio Teheran Braves 6.4 7.8 1.4
Jake Odorizzi Twins 4.3 5.4 1.1
Dylan Bundy Orioles 4.2 5.3 1.1

Per QOP Baseball, the curve’s horizontal movement now sits in the upper echelon–the 95th percentile, for the record–up from the 71st in his big league cameo last year.

Flaherty has also made substantial improvements in tunneling his curveball, a pitch he rather blatantly telegraphed to hitters a year ago. The fastball-curveball pairing on Baseball Prospectus has been polished with a 2.24 inch decrease in release point distance, thanks in large part to horizontal release points that more closely mirror one another.

Statistically speaking, the curveball has been Flaherty’s most improved pitch tunneling-wise.

Jack Flaherty: Changes in Tunneling Metrics vs. Lefties by Pitch Type

Pitch Type Release Distance Pre-Tunnel Max Distance Plate:PreMax Ratio
Pitch Type Release Distance Pre-Tunnel Max Distance Plate:PreMax Ratio
Curveball -1.79 -0.19 2.1
Changeup -0.77 0.09 0.9
Slider -0.28 -0.10 -0.5
Sinker -0.53 0.03 -0.9
Fourseamer -0.52 -0.13 -2.7

So there you have it for Jack Flaherty’s breaking pitches. I hope you enjoyed what began as modest curiosity of mine and promptly spiraled into a less-cursory analysis of the more nuanced facets of pitching in the modern era, where our technological capacity continues to expand at an exponential rate. Maybe I am screaming–or spilling electronic ink–into the empty abyss as I furiously write about expected park adjusted pitch tunneling spin rate optimization above replacement. You know, xPAPTSROAR, you uncultured swine.

My eyes are currently burning, having been vaporized by the artificial light beaming from my laptop. As I continue to download video and data files and storage space dwindles, I acquire the growing sense that said laptop is nearing the point where it will gain sentience and fling me out the window as society collectively watches a Cardinals blogger tumble to the ground with the sheer elegance of a prime Adam Wainwright curveball. Anyways, eat Arby’s. Or don’t. My job description is to supply you with all the insightful baseball analysis your heart desires, not corrupt your arteries with a 500-calorie roast beef sandwich.

If you want an overarching takeaway from this post, it should be that Jack Flaherty is still only 22 years of age and already honing his craft with more precise control, greater movement, and increased deception in attempt to seize the upper-hand against left-handed batters. He obviously still has development and pitch refinement ahead of him, but Flaherty has established himself as one of the more promising long-term products in an organization with an esteemed reputation for churning out young pitching talent.