Logan Gilbert #36 (Photo by Steph Chambers/Getty Images; George Kirby #68 (Photo by David Berding/Getty Images); Bryce Miller #50 (Photo by Alika Jenner/Getty Images); Luis Castillo #58 (Photo by Bryan Kennedy/MLB Photos via Getty Images); Bryan Woo #22 (Photo by Alika Jenner/Getty Images)

The 2025 Seattle Mariners have made the playoffs. October baseball is back in Seattle. The Mariners’ first American League West title since 2001 is theirs to lose with a 98.5% likelihood to win it according to the Grand Salami Time Playoff Simulator. They’ll hold home-field advantage against whoever they could hypothetically face in the Wild Card round—a scenario that gets less likely by the day. They need only two wins to clinch a bye by virtue of earning one of the top two seeds in the American League standings.

They’re in the driver’s seat to earn a bye and avoid the Wild Card round entirely. But what if they don’t? After winning 15 of 16 games after a loss on September 5 in Atlanta, they might just be the hottest team in baseball, though one of their potential postseason opponents in the Cleveland Guardians is staking their own claim for that title.

If the Mariners fail to clinch a first-round bye, there are six possible opponents, though only one American League East opponent, the Boston Red Sox, has a high enough likelihood to consider. The only two teams that have the tie-breaker over the Mariners are their least likely opponents: the New York Yankees and the Toronto Blue Jays.

Fortunately, Bryan Woo appears to have dodged major injury concern and should head a starting rotation that has hit its stride in September, capped by a dominant series in Houston in which the starters (Woo, George Kirby, Logan Gilbert) combined for 17 innings pitched with only 1 run allowed.

So who will it be? Woo should be fresh to start any Game 1 with some hard-earned rest and avoiding the injured list. Logan Gilbert splitter is borderline unhittable (just ask Pitching Ninja). Luis Castillo has pitched three consecutive quality starts—two of which came on the road after an abysmal year outside the friendly confines of T-Mobile Park for most of the season. Can George Kirby break in and steal a spot? It’s unlikely Bryce Miller, who is only five starts back from a lengthy IL stint, will earn the nod at any point, but he should be able to contribute as a potential long reliever. They differ stylistically, but Emerson Hancock might have some advice on that transition, having given up only one hit in his last 4.2 relief innings pitched.

Mariners Starting pitching Stats in 2025: https://www.fangraphs.com/leaders/major-league?pos=all&type=8&team=11&qual=50&stats=sta

Detroit Tigers

Starters: Woo, Gilbert, Castillo

Woo hasn’t pitched against Detroit in 2025, but no matter. He’s your #1 starter going into this matchup, as he should be against most opponents. Pending his return from injury, he should return to the top of the rotation. Gilbert’s 10.1 innings, 3 earned runs and 19 strikeouts could earn him the nod for the Game 1 start to give Woo additional rest if needed. Luis Castillo won both of his starts home and away against the Tigers, allowing 5 earned runs in 12 innings with 11 strikeouts. Chalk should take this matchup against a sputtering Tigers team.

On both Home and Road splits, Woo, Castillo, and Gilbert lead the way. Just looking at their stats at a high level and ignoring matchups, they are the 3 best starters for the Mariners this season.

Houston Astros

The Mariners swept Houston to nearly seal the deal in the AL West, giving Seattle currently a 98.5% chance to win the AL:

Starters: Gilbert, Kirby, Woo

Here’s where it gets interesting—perhaps even sacrilegious. Woo has been the best Mariners pitcher and one of the best in baseball all year, but three pitchers not named Bryan Woo out-pitched him against the Astros this season. Enter George Kirby. Barring one outlier in May 2025, after which he started the year on the IL and hadn’t hit any sort of stride, Kirby dominates the Astros. He always has, and if you’re a Mariners fan, you hope he always will. In 50.1 career innings pitched—and that’s not counting 7 scoreless innings in the 2022 ALDS—he’s struck out 48 and given up only 12 earned runs and is good for a 0.974 WHIP. While Kirby’s track record might just make you consider starting him over everyone for Game 1, you want Gilbert and Woo on the mound as much as possible for a playoff run. Castillo draws the short straw by virtue of Woo’s ace status despite great numbers: 17.1 innings pitched, 19 strikeouts, and only 3 earned runs. Woo slots in for a pivotal Game 3 or, in a sweep situation, there’s the happy possibility of him starting Game 1 in the ALDS and a well-rested Castillo and Gilbert ready for Games 2 and 3.

*https://www.statmuse.com/mlb/ask/george-kirby-against-the-astros*
Kirby has a 2.15 ERA against Houston in his career. He had a shaky start in Houston in May, but he bounced back in last week’s sweep with a 6 inning scoreless gem.

Boston Red Sox

Starters: Woo, Gilbert, Kirby

The Red Sox hit .096 against Woo in 2025, who in 2 starts put up 13 innings, allowing only 2 earned runs and striking out 14. Gilbert struck out 10 in 5 innings in his only start, and Castillo managed a quality start despite giving up two home runs in his one matchup with Boston. Kirby and Castillo are both peaking around the same time, so either could start here, but Kirby’s precision might earn him the nod.

Kirby did not match up against the Red Sox this season, but if you look at his last 4 starts against them dating back to the end of 2023, he gave up a total of 4 runs in 4 games (22.2 IP).

Cleveland Guardians

Starters: Woo, Gilbert, Castillo

Here’s another American League Central opponent who avoided Woo. Gilbert, Castillo, and Kirby all pitched well against the Guardians in their two starts. No reason to shake up the pecking order. They’ve overtaken Detroit for the division lead and the likelihood of a matchup with the Mariners gets lower by the day.

Final Thoughts

Bryce Miller will most likely be the odd man out this post season. He has dealt with two injuries throughout the season and has not been as strong or consistent as the other four with an ERA in the mid 5s. The Mariners may very well bring 4 starters onto the postseason roster (maybe 5 if you count Emerson in the pen). Woo, Gilbert, Kirby, and Castillo will all be pitching in the postseason, opponent and splits-dependent.

At home, expect Bryan Woo, Luis Castillo, and Logan Gilbert on the mound. On the road, it could be more of a tossup and depend on the opponent. If the Mariners play Houston, give the rock to Kirby on the road. Otherwise, let’s stick with Bryan Woo, Luis Castillo, and Logan Gilbert.

Keep in mind, in years past, starters often end up in the pen and pitch in critical moments late into the games. If Kirby is not slotted to pitch for a series, you will likely see him coming out in one of the most important moments of a game.