Zac Gallen closes May with a 7.04 ERA as Arizona falls to Seattle in extras

Zac Gallen finished May with a 7.04 ERA over 6 starts, per Arizona Diamondbacks Stats & Info, as the Diamondbacks fell to the Seattle Mariners in extra innings on May 29 at T-Mobile Park.

What happened

The Diamondbacks dropped the game in the tenth inning, unable to convert on opportunities that, per Yahoo Sports, might have swung the outcome with better timing from either Gallen or the offense. Geraldo Perdomo was a bright spot, breaking out of a funk with a notable defensive play — throwing out a runner in the fourth inning — but the evening ultimately belonged to Seattle.

Gallen, who entered the month as one of Arizona's rotation anchors, was unable to extend what Yahoo Sports described as a quality-start streak. The right-hander's May line, as detailed by Arizona Diamondbacks Stats & Info, reads: 6 starts, 30.2 innings pitched, 37 hits allowed, 24 earned runs, and a 1.57 WHIP. That works out to a 7.04 ERA for the month — a number that, whatever else you want to say about May, does not invite a second look for comfort.

Why it matters

The month-long arc is what makes the May numbers worth examining closely. Arizona Diamondbacks Stats & Info noted that Gallen allowed just 2 home runs over his first 6 starts of the season, then surrendered 8 home runs over his last 5 starts. That is a meaningful shift in the kind of contact he is generating, and it coincides with a stretch in which he has averaged under 5 innings per start in 2026, per the same source.

For a rotation that needs length — particularly in a series against a Mariners club that can grind at-bats — Gallen exiting before the fifth inning on a consistent basis places additional strain on a bullpen that then has to navigate the middle and late innings, and, as Thursday demonstrated, extra frames as well. Under general roster mechanics, pitchers on the 15-day injured list or stretched thin across a road trip leave a club with limited margin for error in the innings that follow an early exit. Thursday illustrated that margin running out.

The season ERA of 5.16, per Arizona Diamondbacks Stats & Info, ranks as the fifth worst in MLB among qualified pitchers. That is not a number that exists in isolation; it is the kind of figure that shapes rotation sequencing and how a coaching staff thinks about workload distribution heading into June.

The numbers

Gallen's May line, per Arizona Diamondbacks Stats & Info: 7.04 ERA, 30.2 IP, 37 H, 24 ER, 1.57 WHIP across 6 starts. His season ERA sits at 5.16, fifth worst in MLB among qualified pitchers. He allowed 2 home runs over his first 6 starts of the season and 8 home runs over his last 5 starts. He is averaging under 5 innings per start in 2026.

Those two home-run figures are worth sitting with side by side. Two in the first six starts is the kind of number that suggests a pitcher commanding the zone and limiting hard contact. Eight in the last five is a different story — one that opposing hitters appear to have started writing somewhere in the middle of the month, and that Gallen has not yet been able to edit.

Where the club stands

Arizona enters Saturday at 86-76 (.531), third in the NL West and 14 games back in the division race. The Diamondbacks are eliminated from both the division race and the Wild Card race, and they arrive at Saturday's game having lost two straight. The Wild Card picture places Arizona fourth in that ranking, 2.0 relative to the cut line — a margin that, combined with the elimination status, frames the remainder of this Seattle series less as a playoff audition and more as a question of professional pride and rotation evaluation.

Saturday's game at T-Mobile Park is expected to feature Ryne Nelson against Bryan Woo. Sunday closes the series with Merrill Kelly opposing Bryce Miller. The club then returns home to Chase Field on Monday, June 1, where Eduardo Rodriguez is the probable starter against the Los Angeles Dodgers, with Emmet Sheehan listed as the probable for Los Angeles.

That home opener against the Dodgers is worth circling. A club that has dropped two straight, is eliminated from postseason contention, and is working through a rotation question at the top of its order will face one of the sharper tests on the schedule when Los Angeles arrives Monday.

What we don't know yet

Yahoo Sports does not address whether the Diamondbacks are considering any changes to Gallen's role, workload, or rotation spot in response to the May numbers. It is not reported whether the home-run spike reflects a mechanical adjustment, an approach shift by opposing hitters, or something else entirely — the numbers are documented, but the cause is not attributed in the material.

It is also not reported what the club's internal assessment of Gallen's trajectory looks like heading into June, or whether the quality-start streak that ended Thursday had been a point of emphasis within the organization. Gallen's next scheduled start is not specified in the material.

What to watch

The rotation question is the one that follows this story into the coming week. With Gallen averaging under 5 innings per start, the games behind him — particularly against a Dodgers club arriving Monday — will test how Arizona's bullpen absorbs the workload. Nelson and Kelly take the mound in the final two games of the Seattle series, and how those outings go will shape how much rest the relief corps has before the Dodgers series opens at Chase Field.

The June calendar will offer the clearest read on whether May was an aberration or a continuation of the trend that Arizona Diamondbacks Stats & Info laid out: 2 home runs in the first six starts, 8 in the last five, and a season ERA of 5.16 — fifth worst among qualified pitchers in MLB. That is the number to watch, not because it tells you what happens next, but because it tells you exactly where things stand right now.