What Has Happened to the Seattle Mariners' Once Great Pitching Staff?

Heading into the 2025 season, the Seattle Mariners' pitching staff was supposed to help carry it to the postseason, however, the group has underperformed and the M's hit the off-day at 33-34 and 4.0 games back in the American League West.

So, what's gone wrong with the team's staff?

INJURIES:

In 2024, the Mariners had four starters make 30 starts or more, but this year has been a totally different story. George Kirby missed the first 1.5 months of the season with right shoulder inflammation and Logan Gilbert is currently on the injured list with a Grade 1 flexor strain. Furthermore, Bryce Miller has been on the injured list twice with right elbow inflammation.

All of this has added up to a taxed bullpen, inconsistent starts from replacements like Emerson Hancock and Logan Evans, and an uneven stretch that has seen Seattle lose eight of its last nine.

In the bullpen, Matt Brash was out until late-April while recovering from Tommy John surgery, and Gregory Santos, who was supposed to be a leverage arm, struggled to the tune of a 5.14 ERA. He's now on the 60-day injured list after undergoing knee surgery.

THE HOME RUN BALL

Mariners' pitchers have surrendered 78 home runs this season, which is the 10th-most in baseball. It's hard to win if you can't keep the ball in the yard. Last season, the team was tied for 17th in home runs allowed, a big difference.

ARI - Gabriel Moreno 3-run HR (5)

📏 Distance: 389 ft
💨 EV: 110.3 mph
📐 LA: 21°
⚾️ 81.3 mph sweeper (SEA - RHP Trent Thornton)
🏟️ Would be out in 30/30 MLB parks

SEA (2) @ ARI (7)
🔻 6th#Dbacks pic.twitter.com/QoviR5nkXP
— MLB Home Runs🚀 (@MLBHRs_) June 11, 2025

REGRESSION

Baseball is a hard game, and sometimes, pitchers don't hold the same success year-over-year. While guys like Gabe Speier have gotten better since last season, other M's pitchers aren't putting up the same numbers.

Miller and Trent Thornton represent one example each from the starting rotation and bullpen.

Pitcher	2024	2025
Bryce Miller	12-8, 2.94 ERA	2-5, 5.73
Trent Thornton	4-3, 3.61 ERA	1-0, 7.71

Those are just two extreme examples, but they point to some major drop-offs from key players that were expected to be big parts of the team's success.

NOT GOING DEEP INTO GAMES

In 2024, the Mariners led baseball in quality starts with 92. That was 12 more than the next-best team, the Philadelphia Phillies. In 2025, the Mariners have 24 quality starts, which is tied for 17th. The pitchers are giving up more runs and not going deep into games, which puts more stress on the bullpen and on the offense, and leads to this:

BULLPEN WOES:

Per @BrittneyAnne08_ of Circling Seattle Sports, an over-exposed and undermanned bullpen has struggled, especially recently:

Since the start of the last homestand, the Mariners' pitching staff (bp+rotation) combined for -0.5 fWAR. Only the A's pitchers produced a worse fWAR over that time.

The bullpen had a 6.34 ERA (ranking 29th) and 1.65 WHIP (28th) over those 15 games.

That needs to change quickly

Since the start of the last homestand, the Mariners' pitching staff (bp+rotation) combined for -0.5 fWAR. Only the A's pitchers produced a worse fWAR over that time.

The bullpen had a 6.34 ERA (ranking 29th) and 1.65 WHIP (28th) over those 15 games.

That needs to change quickly
— britt (@brittanyanne08_) June 12, 2025

The Mariners have a much-needed off day on Thursday but they will be back in action on Friday night against the Cleveland Guardians at T-Mobile Park.

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