Zach Plesac Has Lost What Made Him Break Out

Cleveland’s willingness to trade Francisco Lindor and Carlos Carrasco was the result of two related factors: ownership would not pay anything close to market value for one of the best players in baseball; and the club believed it could maintain competitiveness on the cheap with its core of cost-controlled pitching still receiving pre-arbitration checks. Shane Bieber is the headliner, but breakout seasons from previously unheralded prospects Aaron Civale and Zach Plesac gave the soon-to-be Guardians confidence that they could suppress runs at an above-average clip and hand the game off to a good bullpen.

That plan has not come to pass; Cleveland’s pitching staff ranks just 19th in WAR as of the writing of this piece. But while the bullpen has held up its end of the bargain, sitting ninth in WAR among relief units, the rotation has accumulated just 4 WAR total through the beginning of August.

Much of that can be attributed to the three starters who were supposed to lead the way. Bieber and Civale have been on ice for most season due to a shoulder strain for the former and a finger injury for the latter; both were recently moved to the 60-day IL with the hope of returning in September. Plesac has also missed time, but he hasn’t helped much when he has been on the field. Through his most recent start, he has posted a 4.84 ERA and 5.08 FIP and a strikeout rate of only 15.6%, fifth lowest among starters with at least 80 innings pitched.

Plesac debuted a couple of months into the 2019 season and posted back-of-the-rotation type figures (1.0 WAR over 115 innings pitched) but broke out in the pandemic-shortened 2020, with a 27.7% strikeout rate, 2.9% walk rate, and 2.28 ERA. Jake Mailhot dug into Plesac at the beginning of last August and noted he was relying less on his fastball and more on some improved secondary offerings, most notably his slider and changeup. That overall pitch mix has not changed much this year:

Nor does it change when you split it by handedness:

Despite that consistency between seasons and a penchant for posting neutral platoon splits in the past, his numbers have dipped, in particular against right-handed batters; his wOBA facing same-side hitters has jumped 102 points from last year.

You may have noticed above that Plesac uses his slider almost exclusively against righties. Using that pitch more in addition to his changeup — his best secondary offering going back to the minors — enabled him to put away right-handed batters more effectively, with a .227 wOBA and 28.9% strikeout rate. The slider plays well off of the changeup because they look similar coming out of his hand, then diverge once they reach the plate:

All those gains have been lost this year. Among breaking balls thrown on at least 100 occasions between 2020 and ’21, Plesac’s slider has the 11th-largest decrease in swinging-strike rate, a 9.4-point drop, and went from being comfortably above the league average (about 17%) in that figure last season to right around it this year. The lack of whiffs is partially a product of him leaving his slider in the strike zone more often, going from 42.6% of the time in ’20 up to 46.0% now. Peek at his heatmaps from the past two years and the same narrative emerges: The weapon that buoyed his breakout has often been thrown in locations conducive to hard contact. On balls in play against the slider, the opposition has added almost two ticks to its exit velocity, 9.5 ticks to its hard-hit rate, and 165 points to its wOBAcon.

Control has not been an issue; Plesac locates the pitch in the strike zone almost 10% more than the average starter, and his walk rate across the past two seasons is less than half the league average. Given the decline in swinging-strike rate and increase in zone rate, I would argue it’s his command that’s the problem. Using my called strike probability model, the rate at which he has thrown his slider firmly in the zone (90% to 100% called strike probability) has increased by almost 10%, and the rate that he has thrown it toward the edges of the plate (pitches outside the top and bottom decile in terms of called strike probability) has decreased by 14%.

I wonder if the loss in command is a consequence of Plesac tinkering with the pitch’s shape. Between 2019 and ’20, he added a couple of ticks of velocity to it at the expense of about 4.5 inches of drop, per Baseball Savant. That trend continued this season, with another 1.2 mph gained in velocity and another five inches of vertical movement lost, as well as almost two inches of glove-side wiggle. The total change in movement places 13th in the majors this season among all breaking balls. The change is even more clear when you compare the distribution of all his sliders from ’19 through ’21 (note the movement is relative to a pitch with pure gyroscopic spin):

Plesac’s slider before this season was one of the lowest in terms of spin efficiency, with only 13.4% of the total spin contributing to transverse spin and the rest manifesting as gyro spin. This is reason his slider breaks much more vertically relative to horizontally, and it works well for him because he only imparts a shade over 2,000 rpm of spin on it (almost 500 rpm below the mean for sliders). But in 2021, that spin efficiency has risen to 27.3%; in other words, he’s throwing the slider harder and imparting less drop by doubling up on transverse spin.

On its face, this adjustment is puzzling. Plesac does not spin the ball well, so continuing to leverage vertical movement by means of gyroscopic spin would theoretically be the best course of action. Reducing the movement on the pitch may help him drop it more consistently for strikes, but it has come at the expense of whiffs and quality of contact against.

One possible explanation: Maybe he is trying to leverage the slider more as a pitch to spot for strikes in plate appearances where he has fallen behind? Compared to 2020, he is utilizing it more often in those situations:

This seems plausible, but his lack of success both against right-handed batters and overall would suggest that this shift in approach has not been beneficial.

I will give Plesac some credit; coming off last year’s amazing season, he seems to have spent the offseason making some changes designed to fend off regression and combat the opposition as the league adjusted to his new bread-and-butter pitch. But that may have been premature, and he may be better served leaning more into the gyro-slider as he did in 2020. Cleveland needs that, too. Any hopes of being a contender next season will require not just a healthy Bieber and Civale, but also Plesac rediscovering that slider success.