Jeremy Peña and the Proverbial Rookie Wall

How a mechanical adjustment changed the course of Jeremy Peña's season

As the feeling of opportunity emanated from the stadium on a warm night in Anaheim, baseball’s brightest star delivered a cold message to the player whose opportunity was the grandest of them all: a rookie shortstop whose white cleats only scratched the surface of the shoes he could fill. “Welcome to the big leagues.” Even if Shohei Ohtani did not directly say this message to Jeremy Peña, the point surely came across –– if not before Peña’s first MLB plate appearance, certainly after Peña whiffed at three consecutive sliders from Ohtani, the first three pitches he saw as a Major League Baseball player.

Peña walked to the dugout, likely eager to exhibit more patience in his next plate appearance. After all, only one of the three pitches he saw found the strike zone. In fairness to Peña, however, a slider like Ohtani’s is of the unfamiliar variety to practically all rookies, let alone one in his first MLB game.

If Peña wished for another offering from Ohtani in his next plate appearance, his wish was granted. If he wished for a pitch that would not find the low-outside corner of the zone, well, he would not have been so fortunate. A painted fastball followed immediately by a pair of swung-through sliders sent Peña back to the dugout with everything but answers to the seemingly unsolvable question.

How does one hit Ohtani’s slider?


Facing the Wall


Despite leaving Anaheim without the answer he sought, Peña initially rebounded from his poor MLB debut tremendously, posting a 147 wRC+ in his first 31 career games. Less than two months after the Astros had let their franchise shortstop, Carlos Correa, walk in free agency, Peña appeared to have established himself as a worthy successor, a feat that seemed impossible not too long before.

However, Peña hit the proverbial rookie wall shortly after this stretch of success, and the primary cause of his struggles was identical to the primary cause of his nightmarish first two plate appearances. After his first 136 games, Peña’s xwOBA against breaking balls was .238 on the season and an abysmal .185 since the All-Star break. Peña’s biggest issue was his plate discipline; well-known as a free-swinger, Peña ranked last among qualified hitters in BB/K rate entering the game, drawing approximately one walk for every six strikeouts at that point. His inability to connect with breaking balls certainly exacerbated the issue, as only five players –– each of whom had seen more breaking balls than Peña –– had whiffed on more breaking balls.

Not to be lost amid Peña’s encounter with the rookie wall was his especially poor performance against Ohtani’s slider. The next 23 sliders Ohtani threw Peña after opening day resulted in 2 foul balls, 3 called strikes, 3 batted-ball outs, 5 balls, and a whopping 10 swinging strikes. Just over five months after his Opening-Day start, Ohtani entered his September 10th start against the Astros as mysterious as he was on that warm Anaheim night, at least in the eyes of Peña.


Breaking the Wall


Few were as aware of Peña’s struggles against Ohtani as his manager, Dusty Baker. In fact, Baker slotted Peña into the 8-hole in this game, following a ten-game string in which Peña hit second nine times. A spot that had seemingly become standard for Peña was taken away from him for the first time since the last time Ohtani faced the Astros, when Peña hit ninth just one week prior.

Ohtani’s first pitch to Peña was one that he had never thrown by the time of their first encounter: a 98 mph sinker for a called strike. After taking the next sinker for a ball, Peña found himself in a 1-1 count, ready to do damage.

Damage was done––not to the baseball, but to the bad habit that had plagued Peña for the entirety of his short MLB career. The 1-1 slider that Peña swung through was a microcosm of the last 28 sliders Ohtani threw him. But it wouldn’t be a microcosm of the next one. After watching a 1-2 sinker fall low and inside, Peña laced Ohtani’s 2-2 slider into center field for his first career hit against the right-hander.

The result of Peña’s plate appearance was sweet, but the process was sweeter. After missing the first slider Ohtani offered him, Peña made a mechanical adjustment that would spark a stretch of newfound success. As shown below, his 1-1 swing featured a high leg kick, one that could serve as an asset on well-timed swings but a liability on poorly-timed swings. Regardless, Peña found that a slider as deceiving as Ohtani’s could not be solved with such extraneous motion.

The 1-2 take was likely an easy one for Peña, but not just because of the uncompetitiveness of the pitch. Peña trimmed his leg kick into a quick toe tap, expediting his load and eliminating any movement that could disrupt his timing. He reaped the benefits on the 2-2 pitch and never looked back.


Life on the Other Side


Peña posted a 133 wRC+ for the remainder of the regular season, and if Peña’s strong September opened the door on the rookie wall, his October pushed him through it. One ALCS MVP and one World Series MVP later, Peña had put together one of the greatest postseasons among rookies in MLB history, thanks in large part to his enhanced production against breaking balls.

Jeremy Peña’s Statcast Metrics vs. Breaking Balls

As displayed above, Peña was significantly more productive against breaking balls after his mechanical adjustment, especially considering much of his post-adjustment production came against postseason arms. More notably, Peña’s xwOBAcon increase was even larger than his xwOBA increase, signaling that his surge in production may not have come in the form of improved plate discipline. In fact, while Peña’s biggest issue appeared to be his free-swinging nature, he actually became immensely more productive by swinging at breaking balls more frequently, as evidenced by the table below.

Jeremy Peña’s Plate Discipline Metrics vs. Breaking Balls

Peña’s swing rates against breaking balls, both in the zone and out of the zone, jumped to unforeseen heights during the last two months of the season. The most probable cause of this trend is the improved quality of Peña’s swings against breaking balls, prompting him to offer at more of them. This explanation would suggest that Peña was willing to sacrifice plate discipline to optimize his batted-ball quality.

Optimizing batted-ball quality may not be realistic in the realm of possibility, but if it is realistic among plausibilities, Peña certainly accomplished that. Of the 30 hitters who put more than 25 breaking balls in play after his adjustment, including the postseason, Peña ranked first in xwOBAcon at .518, routing the second-place finisher at .464.


Peña’s Future


The clear caveat to the optimism one may derive from Peña’s September and October statistics is the small post-adjustment sample, one that does not account for the adjustment pitchers likely will make in response to Peña’s new profile. However, although Peña’s superb September and October batted-ball data is likely to regress negatively in the future –– that is, there exists a more reasonable level of production at which Peña’s poor discipline will not be counterbalanced by his batted-ball data as effectively –– Peña does not need superhuman-like batted-ball outcomes against breaking balls to be a valuable asset at the plate.

As long as Peña continues to hit breaking balls at an above-average rate, opposing pitchers will not be able to hide their fastballs from him as frequently. After posting a strong .371 xwOBA against fastballs in his rookie season, Peña is a threat to do even more damage to them as their exposure increases.

If nothing else, Peña’s new swing-rate trend grounds the notion that his mechanical adjustment is reflective of a permanent solution, despite the small sample of data accompanying it, because it exhibits an increase in Peña’s confidence against breaking balls. Although the line between justified confidence and misguided confidence is often ambiguous, a .518 xwOBAcon against breaking balls justifies his confidence to a promising degree.

More than anything, the oscillations of Peña’s rookie season reveal two important, intertwined lessons. First, development is not purely physical. It would have been easy and perhaps accurate to label the 24-year-old rookie as physically developed, but the learning curve of Major League Baseball grants no exceptions.

Second, few opportunities can aid development like facing big-league pitching. Had Peña not faced Ohtani in four separate games before September 10, he may not have encountered the need for a mechanical adjustment. Thanks to the growth mindset that got Peña to this point –– while filling up his trophy case –– the sky’s the limit for the young shortstop.

Photos by Leslie Plaza Johnson/Icon Sportswire and Bernard Hermant/Unsplash | Featured Image by Ethan Kaplan (@DJFreddie10 on Twitter and @EthanMKaplanImages on Instagram)

Aidan Resnick

An aspiring sports analyst, Aidan is a sophomore at the University of Chicago, studying statistics, computer science, and economics. In 2019, he attended the Wharton Moneyball Academy, the Carnegie Mellon Sports Analytics Conference, and the MIT Sloan Sports Analytics Conference, which inspired him to pursue sports analytics. Since then, Aidan has displayed his passion for sports analytics in his newest book, "The Stats Game," where he and his twin brother illuminate statistical tools and debunk myths in sports analytics, in his victorious Diamond Dollars Case Competition project, and in the Resnick Player Profiles, an interactive dashboard that visualizes modern baseball statistics. At Pitcher List, Aidan strives to create content that both builds on preexisting discoveries in the analytics revolution of the 21st century and introduces new methods of analyzing baseball.

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