Lethal Pitch Sequencing with Trevor Rosenthal

The path to the present has been anything but linear for Trevor Rosenthal. A 21st round pick in the masterful 2009 draft, Rosenthal made the leap from Springfield rotation to St. Louis bullpen three summers later. One thing was clear: the kid from Lee's Summit could light up a radar gun. Rosenthal would become Mike Matheny's most trusted reliever by the following season, closing out the 2013 NLCS with a strikeout of Mark Ellis. And while his dream of cracking the big league rotation never came to fruition, Rosenthal has carved out a nice home in the backend of the bullpen. As of this writing, he needs just 13 saves to surpass Todd Worrell for third place on the Cardinals all-time list.

That's not to say that things have always been smooth sailing for Rosenthal. 2016 was easily the worst year of his major league career, with the rock bottom point being a three-run walk-off blast off the bat of Adam Lind, forcing Matheny to hand the reigns to Seung-Hwan Oh as closer. It was only a collapse from Oh that put Rosenthal back in the ninth last month, but he has been nothing short of dominant this year.

Granted, a good portion of his recent success is likely the virtue of a healthier 2017. Inflammation in his throwing arm's rotator cuff shelved Rosenthal for nearly two months last year and a right lat strain sustained in Spring Training forced him onto the disabled list to open up this season. His average fourseam fastball velocity jumped from 97.71 mph in 2016 to 99.14 this year. Naturally, Rosenthal's "raw stuff" would improve when he isn't nursing nagging injuries throughout a 162 game season.

Even if the injury bug (or its absence) played a significant role in his turnaround, something else must have changed to prompt the Rosenthal we have witnessed over the past several weeks, arguably the greatest iteration of him we have ever laid eyes upon. Yes, Rosenthal was dominant in 2013 and 2015, but not to this extent.

One thing I noticed right away is that we are talking about peak Rosenthal only when he is facing righties, not lefties. 2017 has been his best season by wOBA against right-handed batters and his second worst against left-handed opponents.

That led me to see if Rosenthal was attacking righties differently in 2017, which was the moment I found it: the slider. Prior to this season, Rosenthal only used the pitch 6% of the time against righties. Compare that to its 21% usage rate this year and I was on to something. The changeup was his go-to secondary pitch against righties pre-2017, especially with two strikes. Fast forward to this season, where the fastball and slider in tandem have formed a devastating put away combo.

Using Baseball Savant I was able to access data from every single pitch Trevor Rosenthal has ever thrown in the Statcast era. I filtered out all matchups against lefties and looked at each fastball Rosenthal has pitched over the past two years, excluding those that ended the plate appearance. Most importantly, I created a column showing what type of pitch the next offering was.

It's evident that the slider has become his favorite put away pitch alongside the rejuvenated fastball, and for good reason.

One might argue that–given its superior effectiveness–the slider should have been the fastball's main partner in crime all along. Take a look at what BrooksBaseball.net has to say about Rosenthal's three primary pitches. (Note that all stats in the table below are from 2017 alone.)

While the release points between the fastball and slider aren't perfectly identical, they both look much more similar coming out of the hand than the changeup. That keeps right-handed hitters wondering: what's coming next? Triple digit heat with arm-side movement or a pitch 12 miles-an-hour slower that tails in the opposite direction.

Trevor Rosenthal vs. Anthony Rendon

(I want to acknowledge former Viva El Birdos writer and pitching maestro Joe Schwarz, who wrote about this exact same at-bat following Rosenthal's season debut in April.)

BrooksBaseball.net

After falling behind in the count 2-1, Rosenthal comes back with a 99.39 mph fastball (following a slider on 1-1) that Rendon can't catch up to. Now even at 2-2, Rosenthal goes with a 99.10 mph fourseamer, but this time middle-up rather than up-away.

Place yourself in Rendon's shoes for a moment. He has just seen back-to-back fastballs up in the zone, and 12 of Rosenthal's 13 pitches on the night have been fastballs. He gears up for another heater as the pitch leaves Rosenthal's hand. The sixth pitch of the at-bat is following the same path to the plate as pitches four and five when...it suddenly drops, catching the black for strike three. Rendon tosses his bat in disgust. Rosenthal strolls off the mound. Three batters. Three strikeouts.

The aforementioned Joe article featured the gif-ing wizardry of Twitter user @cardinalsgifs, who was gracious enough to grant me permission to use his excellent work.

@cardinalsgifs

You should immediately notice how all three trails (the final three pitches of the at-bat) appear to be heading on incredibly similar courses to home plate in the first few frames of the gif.

TexasLeaguers.com

By the time the batter recognizes any meaningful deviation between the two types of pitches (around 20 feet away from the plate, as shown in the image above), it is much too late for the hitter to react and adjust accordingly. And with a pitcher who throws as hard as Rosenthal, that time at the batter's disposal only dwindles.

I want to give a huge thank you to @cardinalsgifs, along with the wonderful people at Baseball Savant, BrooksBaseball.net, and TexasLeaguers.com. Without them, a piece like this could never happen.