Arizona Diamondbacks 2024 Player Review: Zac Gallen

The Diamondbacks' ace starter pitched well, but battled inconsistencies and injury in 2024.

Sep 15, 2024; Phoenix, Arizona, USA; Arizona Diamondbacks starting pitcher Zac Gallen (23) on the mound in the first inning during a game against the Milwaukee Brewers at Chase Field. Mandatory Credit: Allan Henry-USA TODAY Sports / Allan Henry-Imagn Images

This article is part of a series chronicling the individual seasons of players who appeared for the Arizona Diamondbacks in 2024.

Reviews for players who still have rookie eligibility for 2025 will appear in our prospect season reviews. Players are presented in the reverse order of their aWAR, an average of Baseball-Reference and FanGraphs WAR.

RHP Zac Gallen

2024 Contract status: Arb 2, $10.011 million

Zac Gallen 2024 and career statistics / Alex D'Agostino | Diamondbacks On SI

Since Arizona Diamondbacks right-hander Zac Gallen's emergence as the ace of the staff in 2022, expectations have been laid heavily on his shoulders. In 2023, he had what many thought of as a "down" season, despite pitching to a solid 3.47 ERA, a career-high in innings, and winning 17 games.

So it was somewhat expected to see Gallen re-emerge as a Cy Young-level pitcher in 2024, despite less downtime from the 2023 run.

The ace got off to a solid, though not eye-popping start. He debuted with five innings of one-run baseball on Opening Day, then delivered a brilliant six-inning, three-hit shutout against the New York Yankees.

Things looked to be clicking towards Gallen returning to ace form, but he stumbled a bit in April, including a five-run, nine-hit affair at the hands of the San Francisco Giants. On April 26, Gallen left in the sixth inning of a game against the Mariners with hamstring tightness.

He avoided the Injured List, but his next start was pushed out until May 7, which began a brilliant month for the ace. Gallen delivered four straight Quality Starts, including a six-inning, one-hit shutout against the Reds, and pitched into the seventh twice.

But May ended on another down note for the righty. After facing just two batters, Gallen exited in the first inning against the Mets without recording an out. This time, he couldn't avoid the Injured List, landing on the 15-day IL with a hamstring strain, in all likelihood an aggravation of his prior inury.

Gallen returned on Jun 29, in the midst of some of Arizona's deepest rotational struggles. He instantly tossed a brilliant shutout against the Athletics, going six innings, allowing one hit and one walk while striking out seven over six innings on just 77 pitches.

But his season began to deteriorate after that start. In six July starts, Gallen recorded just two Quality Starts, and was blown up for six earned runs at the hands of the lowly Blue Jays on July 14 over just 3.2 innings.

Gallen's outings began to look more and more labored, and his command took a dive, as he pitched to a 5.10 monthly ERA, with his WHIP surging to 1.25.

August was a similar grind. A six-inning shutout against the Red Sox to help complete a Fenway Park sweep was bookended with a pair of shaky starts.

After being hit around by the admittedly powerful Los Angeles Dodgers on August 30, his monthly ERA sat at 4.94, and his season total was inching dangerously close to the 4.00 mark.

But with the D-backs in the heat of a tough playoff race, Arizona's ace put forward his best month of the season in September when the club needed it most.

Over five September starts, Gallen posted a 4-0 record, tossed two Quality Starts, and allowed four or fewer runs in each of his appearances.

It didn't look spotless, and he only managed five innings three of the five starts, but he did strike out 38 batters over 27.0 innings, and he held opponents to a .175 average on the month, contributing to his season-best 2.67 September ERA.

His season culminated with one of the best starts of 2024, striking out a season-high 11, while allowing just one run on two hits against the San Francisco Giants at Chase Field.

His late-season surge helped him return his K/9 to 9.49 (his highest since 2021), his 3.38 FIP suggested he might have been slightly better than his 3.65 ERA, but his statcast xERA of 3.94 suggests the opposite.

But although his numbers did end up looking very respectable, with a 14-6 record as a bonus, Gallen simply didn't look like the pitcher D-backs fans had come to know. His command was shaky, and he walked 3.28 batters per nine, the most in his past three seasons by nearly a full walk.

Part of that was location struggles, which contributed to a lack of efficiency. His first pitch strike percentage dropped all the way to 58% from his previous career 65%. (League average is 61%).

In 2023, Gallen threw just over 15 pitches per inning (95.5 per start), but threw nearly 17 per inning in 2024. While he averaged fewer pitches per start (89.5) in 2024, he also averaged just over five innings per start this season, down almost a full inning from his 2023 average.

Gallen's diverse arsenal saw fluctuations in both velocity and movement, but the most notable difference was in his fastball run value. In 2023 his fastball had a +19 run value, ranking in 98th percentile. In 2024 his fastball had a -4 run value, ranking in the 25th percentile. He simply was not able to command the pitch in 2024, either missing and falling behind in counts, or leaving it in bad spots.

His knuckle curve took a step backwards as well. Profiling as one of his best knockout pitches in 2022 and 2023, the pitch was worth an incredible +27 run value in 2023 but dropped to +13 in 2024.

Now, this is not to say his curve became a "poor" pitch by any means, nor that Gallen was not effective in 2024. He still pitched to impressive numbers, was a leader in Arizona's rotation, and came up big in big moments as the season grew long.

Something was clearly off with the ace's fastball in 2024. Whether it was mechanical issues, lingering fatigue from the 2023 season, mental struggles or simply injury that dampened Gallen, he didn't pitch as well as he's shown the ability to, allowing his highest season WHIP (1.26) since 2021.

Perhaps the extended absence of No. 2 starter Merrill Kelly, or the underperformance by the rest of the rotation (excluding Ryne Nelson) played a role in his inconsistency, with a steep gap between Gallen and most of the rest of the rotation.

That said, he was still worth 2.8 aWAR, led D-backs starters in ERA, and utilized his diverse arsenal and impressive intellect to string together some of the most electric performances by an Arizona starter through 2024.

He had plenty of head-scratching outings, but ultimately delivered down the stretch.

2025 Outlook

2025 status: Arbitration 3, $14.1 million estimate per MLB Trade Rumors

Despite some of the questions surrounding Gallen, he will still be set to remain the ace of the rotation in 2025. An extended offseason, coming off just 148.0 innings of work could be the recipe for a bounce-back year.

Gallen is in his final year of Arbitration, and will become a free agent after the 2025 season. With that comes an entire article's worth of speculation regarding his future with the D-backs (or another club). Certainly, Gallen will command a massive payday after 2025, and it will likely be difficult for Arizona to compete with external offers.

There is, of course, a chance Gallen is moved at the 2025 Trade Deadline, though that seems a very distant possibility at most. Especially since the Diamondbacks would be virtually assured a late first round compensation draft pick once Gallen rejects a qualifying offer. In all likelihood, Gallen spends his 2025 in the desert before seeking a bigger payday in free agency.

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