A New Ace Emerges in Seattle’s Rotation

Over the past half-decade or so, the Mariners have emerged as one of the game’s truest pitching labs. Not only do they have the innate ability to pluck an elite, high-leverage arm out of thin air, but their ability to draft and develop starting pitching is severely underrated. Four of their five rotation mainstays were drafted and developed, with Luis Castillo being the only one acquired via trade in 2022. The team’s “outright” ace was drafted in 2018 and made his debut in 2021. George Kirby was drafted in 2016, debuting in 2022. Bryce Miller was also drafted in 2018 and debuted in 2023. The team’s newest emerging ace, Bryan Woo, was drafted in 2021 and debuted in 2023.

A New Ace Emerges in Seattle’s Rotation

But it doesn’t just stop there. The team’s two main injury fallback options, Logan Evans and Emerson Hancock, were also drafted and developed by Seattle.

What does this mean in the long run?

This ability allows Seattle to take chances on their prospects and allocate funds when they do elsewhere. We’ve seen this a couple of times recently, with both Cal Raleigh and Julio Rodriguez signing long-term extensions. It also allows them to save their prospect capital for important offensive additions.

But above all else, it allows them to compete with teams that are better than they are.

Bryan Woo Continues His Dominance

Coming into 2025, it was the common thought among fans that Castillo had passed the role of the staff’s ace to Gilbert. Gilbert, an All-Star in 2024, led the staff in both innings pitched and strikeouts. His ERA of 3.23 was second only to Miller (amongst Mariners who started 25-plus games).

Injuries have limited Gilbert to just 12 starts as of July 15th, and when he has pitched, he has looked noticeably off, although this could be due to him needing time to adjust.

Regardless of that, the one constant in the Mariners’ rotation this year has been Bryan Woo. Woo picked up where he left off in 2024. Leading the Mariners in ERA with a 2.89 mark across 22 starts, Woo has continued to improve to start 2025.

Nominated for his first All-Star game in 2025, Woo only trails Castillo in innings out of the Mariners’ rotation.

What’s different in 2025?

For starters, he’s mixing his pitches differently. His fastball use is down, but he’s supplemented that with his slider and sinker. He’s walking more batters, but he’s also striking out more batters than he ever has, already eclipsing his total amount of strikeouts in 2024. This suggests that he’s finding less of the zone, but in a good way.

Kirby also had a similar “problem” of throwing too many strikes. He was catching too much of the plate, which can make it easier for hitters to ambush you. By missing more of the zone, opponents are batting lower and slugging less against Woo than ever before. The swing and miss aspect of his game is also higher than ever, with his K% up, swing and miss up, and whiffs up.

Woo has cemented himself as the Mariners’ best pitcher in 2025, and moving forward, he can take his position from a mid-rotation arm to a potential ace.