Diagnosing a puzzling down year for Zac Gallen

In July 2019, Diamondbacks general manager Mike Hazen made the most fascinating trade of his tenure when he dealt highly-touted shortstop prospect Jazz Chisholm to the Miami Marlins in exchange for right-handed pitcher Zac Gallen. Most fans had the same reaction: Zac who?

Gallen’s anonymity didn’t last long. Just over a year after the D-backs acquired him, Gallen broke the MLB record for most consecutive games allowing three or fewer earned runs to start a career. In 2020, he earned down-ballot Cy Young votes with a 2.75 ERA, 1.11 WHIP and a 28.2 percent strikeout rate.

That brings us to 2021, when Gallen hoped to showcase his success over a full season for the first time. Unfortunately, he took a step back, posting an essentially league-average 4.30 ERA and 1.29 WHIP over 121.1 innings. Let’s try to diagnose the problem.

Like almost everyone on the 2021 roster, Gallen caught the injury bug — three times, actually. The first incident occurred on March 10 when Gallen experienced discomfort in his right forearm after taking batting practice against a pitching machine (National League DH, anyone?). He suffered a stress fracture in his right lateral forearm, which delayed his first start of the regular season until April 13.

Gallen looked like himself in his first five starts, highlighted by a seven-inning, one-hit shutout in Atlanta in late April. 

On May 12, Gallen was scratched after a precautionary MRI revealed a minor sprain of part of his UCL. It was a rough day for Diamondbacks Twitter.

It was fun while it lasted…. see ya in 2023

— Stevitude (@stevitude) May 12, 2021

Fortunately, Gallen avoided Tommy John surgery and returned a little more than a month later, only to land back on the injured list shortly thereafter due to a strained hamstring. Unfortunately, when all was said and done, Gallen just wasn’t the same pitcher in 2021 as he was in 2020.

It’s not unreasonable to blame three separate IL stints as the root cause of Gallen’s down year, but it’s worth digging into the numbers to pinpoint exactly why he wasn’t as effective. A few key differences stand out.

First, the velocity gap between Gallen’s fastball and changeup decreased by 1.2 MPH from 2020 to 2021. Specifically, his average four-seam velocity increased from 93.2 MPH in 2020 to 93.4 MPH in 2021, while his average changeup velocity increased more significantly from 84.8 to 86.2 MPH in 2021.

While those numbers aren’t alarming, it’s no secret Gallen relies on his fastball/changeup combination heavily to be effective. Any adjustment that leaves those two pitches looking more alike probably isn’t a good one.

That may explain why the whiff rate on Gallen’s prized changeup decreased from 23.1 percent in 2020 to 16.3 percent in 2021, despite no significant changes in the movement of the pitch. Location is also a factor, as Gallen landed his changeup on the edges of the zone about 10 percent more in 2020 than 2021.

In the end, his changeup putaway percentage decreased from 30.9 percent in 2020 (the highest of any of his pitches) to 17.2 percent in 2021 (the second lowest of all his pitches). Restoring the effectiveness of his changeup will be a big step toward getting back on track for 2022.

Filthy changeup from Zac Gallen pic.twitter.com/V5rNBSG0Mj

— Nick Pollack (@PitcherList) August 6, 2020

Fortunately for Gallen, he has more weapons than just his changeup and fastball. His second-best putaway pitch in 2020 was his curveball. In 2021, it was his best, even boasting several more inches of movement than it has in the past.

Out of 63 plate appearances against Gallen that ended with a curveball in 2021, 28 resulted in a strikeout. His 17.9 percent whiff rate on the pitch was also a career-high.

Curiously, Gallen used the curveball just 12.5 percent of the time in 2021, compared to 20.1 percent in 2020. Perhaps that is injury-related — Gallen mentioned that he initially only felt his stress fracture when throwing his curveball. Nonetheless, assuming he’s fully healthy now, Gallen should consider using it more.

Zac Gallen, Pretty 81mph Curveball. 🌈 pic.twitter.com/ZhJzDncY5E

— Rob Friedman (@PitchingNinja) August 1, 2020

To throw more curveballs, Gallen is going to have to cut back on other pitches. One of those could be his four-seam fastball, which Gallen threw 55 percent of the time in 2021 compared to just 39 percent in 2020. Notably, his fastball got a lot more of the zone in 2021 than it did in 2020.

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Considering all that red in the middle of the zone, it’s impressive that Gallen still held hitters to a .217 batting average and a .366 slugging percentage on his heater in 2021. Nonetheless, the hard-hit rate on his four-seamer skyrocketed from 26.2 percent in 2020 to 47.1 percent in 2021. He probably doesn’t need to throw it as much as he did last year.

Speaking of fastballs, Gallen tinkered with a new cutter in 2021 to complement a slider/cutter hybrid that he’s thrown for years. He discussed his new weapon last April in an interview with the Pitching Ninja.

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Gallen primarily used his cutter to jam lefties inside. Here’s an example against Brandon Belt.

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The strategy was fairly successful, as left-handed hitters posted a reasonable .314 weighted on-base average (wOBA) when putting the pitch in play. A .226 expected wOBA indicates Gallen could have done even better.

The real issue with the cutter came against righties. In albeit a very small sample (just 53 pitches), right-handed hitters went 4 for 9 with a homer and a double when they put the cutter in play. Tommy Pham got the better of it here.

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Ultimately, it’s hard to say if the new cutter helped Gallen in 2021, but it probably didn’t hurt him either.

That leaves us with Gallen’s final offering, a slider/cutter combo (slutter?) that has confused pitch identification software for years. Historically deemed a cutter by Baseball Savant and a slider by Brooks Baseball, Gallen generally uses the pitch low and away to righties.

Zac Gallen, Filthy 87mph Slider. 😷 pic.twitter.com/OuJmzNZtIh

— Rob Friedman (@PitchingNinja) September 15, 2019

Likely due to the addition of a true cutter, Gallen threw the slutter more like a slider in 2021. It had a few more inches of vertical break, and it was a tick softer than in past years.

Nonetheless, results against the offering remained essentially unchanged. In 2020, opposing batters hit .328 with a .516 slugging percentage when they put the pitch in play. In 2021, they hit .250 with a .667 slugging percentage against Gallen’s updated version. Whiff rates on the pitch were almost identical from 2020 to 2021.

Despite throwing the slutter just eight percent of the time in 2021, Gallen allowed six of his 18 homers on that pitch. Given that Gallen’s curveball has always been more effective than the slutter against righties throughout his career, it’s worth questioning if he should do away with the slutter altogether.

All numbers aside, it’s easy to understate the importance of health and continuity for Gallen moving forward. He may not need to change much. He may just need to stay on the field, and the rest will take care of itself.

But one thing is certain: If anyone can help Gallen maximize his skillset, it’s new pitching coach Brent Strom. It will be fun to watch what that duo comes up with in 2022.

Follow Jesse Friedman on Twitter