D-backs Have Clear Choice to Make Between 2 Free Agents

Between Merrill Kelly and Zac Gallen which arm should the Arizona Diamondbacks prioritize re-signing?

Arizona Diamondbacks starting pitcher Merrill Kelly (29) looks on against the Texas Rangers during the first inning in game two of the 2023 World Series at Globe Life Field on Oct. 28, 2023, Arlington, Texas. | Joe Rondone/The Republic / USA TODAY NETWORK via Imagn Images

The Arizona Diamondbacks' starting rotation is in a state of disarray after a tumultuous 2025 season.

With free agency looming, two former D-backs arms will be on the market — Zac Gallen and Merrill Kelly. Gallen finished his season with Arizona, while Kelly departed via trade to the Texas Rangers at the Deadline.

With both having strong connections to the D-backs, both are candidates to return to Arizona on a reunion deal in 2026 — to differing degrees of realism, of course.

Granted, they're very different pitchers, at different stages of their career. But there's one that makes much more sense for what Arizona needs than the other.

Comparing Zac Gallen, Merrill Kelly

Arizona Diamondbacks pitcher Zac Gallen (23) throws to the Texas Rangers in the first inning at Chase Field on Sept. 3, 2025. | Rob Schumacher/The Republic / USA TODAY NETWORK via Imagn Images

Let's start with Gallen. The 30-year-old right-hander rose to ace status after an exceptional 2022 season saw him post a 2.54 ERA and 3.05 FIP over 184 innings, before he helped lead his club to the World Series a season later.

But following that career season, Gallen began to enter a steady decline after the 2023 All-Star Break. From the second half of 2023 on, he's posted a 4.25 ERA, culminating with a 4.83 number for the 2025 season.

If not for a late-season surge post-Deadline, Gallen might have finished as one of the worst qualified starters in baseball. For his major league career, he owns a 3.58 ERA and 3.65 FIP over 1,007.1 innings.

Kelly, meanwhile, is entering his age-37 season. He pitched to a 3.22 ERA over 22 starts with Arizona in 2025 — on pace for the best full-season ERA of his career. He stumbled a bit once heading to Texas, posting a 4.23 number in just 10 starts with the Rangers.

Kelly's never been thought of as a true ace, but has been a very reliable arm, serving as an anchor in the D-backs' rotation for the past handful of seasons.

Kelly owns a career 3.77 ERA and 3.99 FIP over 1,008.1 major league innings. However, he also has four full seasons worth of innings in Korea from 2015-2018.

So which pitcher better suits Arizona's needs? The choice is relatively clear, given the circumstances.

Why Merrill Kelly is Better Re-sign for Diamondbacks

Aug 24, 2025; Arlington, Texas, USA; Texas Rangers starting pitcher Merrill Kelly (23) comes off the field after he pitches against the Cleveland Guardians during the first inning at Globe Life Field. Mandatory Credit: Jerome Miron-Imagn Images | Jerome Miron-Imagn Images

Between the two pitchers, Gallen has the higher ceiling and more of an ace pedigree. Kelly, however, has been the more consistent arm through the years.

Meanwhile, Gallen is expected to test free agency, declining the $22 million Qualifying Offer. That means Gallen and his agent (the famed Scott Boras) are likely out for both a high dollar amount and multiple years.

Committing that much money and that many years to a pitcher who has been in steady decline for two years is a huge risk, even if he did begin to look more like his old self at the end of the 2025 season.

Kelly would take significantly less to get back, both in dollars and years. With a reduced payroll likely to hit Arizona in 2026, the D-backs could invest a (relatively) low amount in Kelly, knowing both the potential risk and reward would be lower.

Not that Kelly would cost pennies, but it certainly would be nowhere near a $100-plus million commitment.

Granted, there's a risk in returning Kelly, as well. His numbers tailed off as 2025 progressed, he's struggled with a persistent cramping issue and a major shoulder injury, and is in his late 30s, when regression can hit even the best arms hard and fast.

Related Content: What is Merrill Kelly's Market Value to the Diamondbacks?

But as a whole, knowing Kelly's "worst" is likely still a passable starter (while Gallen's worst can be difficult to watch), it makes it easy to walk away from the boom-or-bust gamble it would be to pay Gallen.

Corbin Burnes is also expected to make his return around the All-Star Break. If he remains as good a pitcher as he has been (which is not a guarantee), the D-backs won't need an ace as much as they need rotational depth and veteran sturdiness. Kelly is a better source of both, for a better price.