How the Mariners should line up their playoff rotation with October approaching

The Seattle Mariners have been cruising to the postseason, with their 10th straight win on Tuesday also marking an important milestone for Cal Raleigh. Now, they have a 97.3 percent chance to make the postseason, per FanGraphs.

It seems dangerously premature to start counting the Mariners' eggs before they hatch, but Seattle has hit a real stride and could start planning for the postseason soon. The first order of business would be to figure out the layout of the rotation in October, a non-trivial task with a team this talented.

What the Mariners' playoff rotation should look like

Fortunately, finding the leader for this group of starting pitchers is a freebie. Bryan Woo has been one of the best pitchers in the American League this year, with a 3.02 ERA and a 3.55 FIP over his 29 starts — he also leads the all Mariners pitchers in rWAR (4.0).

We've been getting a glimpse of what Woo is capable of since he debuted in 2023 and in his first full big league season, he has lived up to the hype and more. He was called up a year too late to join his fellow teammates in Seattle's 2022 playoff run, but his dominant stuff should more than make up for his lack of experience.

Further down is where the waters get murkier. Logan Gilbert hasn't been as outstanding as he was last year, but he's far from ineffective. He has a 3.53 ERA for the season and a 3.86 ERA since the start of August, significantly better than George Kirby (5.02) and Luis Castillo (5.58) have pitched in that span.

There should be some concerns about Gilbert's ability to pitch efficiently as this season, he has shown a tendency to need nearly 90 pitches just to get through six innings. Most recently, he couldn't even make it to the sixth inning after throwing 95 and 89 pitches in his last two starts, respectively.

Still, he has elite whiff (33.3, 94th percentile) and strikeout (33.3, 95th percentile) rates while walking just 5.9 percent of batters and displaying nasty stuff. As far as No. 2s in playoff rotations go, one could do much worse than Logan Gilbert.

For the third spot, George Kirby's raw upside is enough to edge out Luis Castillo and Bryce Miller. Kirby has had some brutal outings, especially on the road, and didn't look too good against the Cardinals on September 9th. But against the Angels on September 14th, he spun a gem, striking out 14 batters in 6.1 innings while giving up just two earned runs. Funnily enough, it was the second time he punched out 14 Angels in a start this season.

Kirby still has some of the best stuff in baseball and his postseason record speaks for itself – his seven inning scoreless start against Houston in the 2022 Division Series was an incredible showing for a rookie. So despite some bumpy starts here and there, the unhittable George Kirby of seasons past is still in there somewhere.

With Bryce Miller as the only other option, Luis Castillo is the clear choice for the final spot in the rotation. His 3.63 ERA over 31 starts is a respectable figure for the season, but his recent numbers should raise alarms. As mentioned earlier, he has a 5.58 ERA since the start of August and his velocity has decreased significantly, drastically limiting the effectiveness of his four-seam fastball as of late.

Castillo's last two starts against the Angels and Braves have resembled the Castillo that the Mariners traded for in 2022, but his scoreless start on Thursday was his first since July 18th. Nonetheless, he's a big league veteran with the most postseason innings and a 1.83 ERA in October.

Furthermore, it just hasn't been Bryce Miller's year. Even after a few injured list stints, he never seemed to find his footing and saw a sharp decline in his numbers from his breakout sophomore campaign.

He hasn't seemed prepared to pitch in the regular season, let alone the playoffs and it's an issue he and the organization will have to work on in the offseason if he wants to remain in the rotation. There's a chance that he gets some work coming out of the bullpen which is often where the last guy in the rotation ends up anyway. But given his 5.58 ERA and 5.14 FIP, it'd be foolish to rely on him to start games.

Again, there's still a week-and-a-half of regular season games left, and the Mariners have a tendency to find new and creative ways to fall short. But barring a calamitous collapse, Seattle will be in the postseason this year. How they do will depend on several factors, but the most important one is their starting rotation.

It has been the foundation of their recent success and although it hasn't been quite as prolific on paper this year, the sheer talent on the roster has only improved. They've all but passed the tests of the regular season – now it's time to see how they fare when the pressure is on.