Searching for a weakness in Chicago Cubs ace Jake Arrieta

After being named the 2015 National League Cy Young Award winner over Los Angeles Dodgers co-aces Clayton Kershaw and Zack Greinke, expectations have never been higher for Jake Arrieta. Of course, a contributing factor to these expectations is the offseason excitement surrounding Arrieta's team, the Chicago Cubs, who has made significant, roster-defining splashes (Jason Heyward, Ben Zobrist, John Lackey) in the free agent market. After finishing three four wins (post-publishing edit: I mistakenly thought the Cubs won at least one game in the NLCS) short from an appearance in the World Series, it is safe to say team president Theo Epstein can taste the opportunity of finally bringing the title back to the north side of Chicago (a feat he helped accomplish while he was general manager for the Red Sox back in 2004).

2015 PitchF/x basics (via BrooksBaseball.net)

Remember: Regarding horizontal movement in right-handed pitchers, a negative value means arm-side movement, whereas a positive value means glove-side movement.

It does not take a degree (or even a minor, for that matter) in PitchF/x analytics to appreciate Arrieta's PitchF/x table. There are many aspects to talk about, but most notably, Arrieta's average sinker velocity topped that of his fourseam fastball (which is highly unusual). Even further, Arrieta possessed the fastest starting pitcher sinker in all of baseball last season, eclipsing Edinson Volquez, the second place finisher, by a full MPH. It does not stop there, either, as Arrieta also captured the crown of MLB's fastest starting pitcher slider (ahead of the flame-throwing World Series runner-up Jacob deGrom).

Moving one column to the right, one can somehow appreciate Arrieta's dragless horizontal movement even more when also taking into consideration his "cross-fire" mechanics and subsequent ability to hide the ball quite well all the way up to his release point. The differences in horizontal movements between his sinker and slider (17.07 inches, on average) or his sinker and curveball (21.8 inches, on average) make the two combinations borderline lethal to opposing hitters.

Pitch values, while they have come a long way, are still a work in progress. However, I do think we will eventually get to a point where they will be much more widely appreciated, especially as Statcast is fine-tuned and as there becomes a greater understanding behind the importance of pitch sequencing. Regardless, it is still impressive to consider that all five of Arrieta's pitches were in the National League top ten on a rate basis (value per 100 pitches), with his two most frequently used pitches—the sinker and slider—placing first and third, respectively.

Admittedly, up to this point, I have provided nothing but praise for the 29-year-old Arrieta, and I have not yet fulfilled the title of the article. Frankly, there usually are no glaring weaknesses in a pitcher of Arrieta's caliber, but as with any pitcher (even the otherworldly Kershaw), there are still areas, however small they may be, hitters can exploit.

Location, location, location

As you can see from the beautiful new heatmaps on Daren Willman's BaseballSavant.com, Arrieta was successful in 2015 by living on the outside corner to opposing hitters, regardless of handedness. Now, this should not be surprising considering his two best pitches (slider to righties, sinker to lefties) naturally fall into those general zones.

In terms of vertical location, the y-axis values for both cores (the dark brown-orange spot in the center of the heat signature) were 2.25. Generally speaking (considering the y-values run from 0 to 5), the y-value for the vertical middle is 2.5, so based on the two cores seen above, Arrieta usually located his pitches one deviation lower than the middle.

Okay, so what does any of this mean? I looked at Arrieta's game logs on Baseball-Reference to see which games he "struggled in" during the 2015 season. I use quotation marks because never once did Arrieta allow more than four earned runs in a start last year. However, he did allow four earned runs on four separate occasions (all before the All-Star break) and three earned runs (plus three unearned runs) on one occasion. Using Baseball-Savant's search function, I generated heatmaps for each one of these starts, which in turn gave me the following values:

While game-to-game pitch location is highly variable (given that we are dealing with much smaller sample sizes), in each one of these starts Arrieta's vertical core, for both types of batters, fell at least one deviation higher than what we saw in his overall 2015 maps above. Again, I am not, by any means, trying to draw a concrete conclusion here, but rather simply be descriptive in nature.

I understand that this is not necessarily intuitive because most pitchers tend to have more success when they pitch down in the zone, but given the complexity and quality of Arrieta's repertoire, he should be able to have success up in the zone as well. Yet, as shown in the BrooksBaseball.net chart below, when hitters put the ball in play on pitches up in the zone (boxed in yellow), 28% of the batted ball types were line drives, not necessarily a recipe for success for pitchers. Thus, going forward, if Arrieta's pitches ever stray upward in the zone (something they rarely did in 2015), it would be wise for hitters to pounce on them (read: you're simply not going to make good contact on his sinker or wipeout slider down and away).

Another quick thing to note from the table is the variability in the x-axis columns, specifically with right-handed batters. Three of the five cores landed on the inside portion of the zone to righties, while two of them fell right down the middle of the zone—a stark difference from living on the outside corner as we saw in the very first heatmap.

Bottom line

Jake Arrieta is a tremendous pitcher. I respect him oh so much. He has one of the league's very best repertoires and a deserving Cy Young Award to show for it. However, he was in a near-historic "zone" last year, for an extended period of time. Yet, in his two October starts after the Wild Card game, his arm showed clear signs of fatigue, allowing four earned runs in each start and not making it out of the sixth inning in either start.

While ZiPS deservedly projects him favorably (see below), I cannot help but wonder if he will enjoy such an extended "zone" in 2016. How will he rebound from 248.2 innings pitched last season (his previous high was 156.2)? As a fan of the rival St. Louis Cardinals, I cannot wait to find out.

2016 ZiPS Projection (via @DSzymborski)