Mariners' choice for Game 1 playoff starter is becoming clearer every fifth day

Last season couldn't have gone much better for the Seattle Mariners rotation, as it finished first in a whole host of categories including quality starts, ERA, walks, WHIP and opposing team batting average. This year has been a bit more of a mixed bag, although it hasn't been helped by an injury bug which impacted 60 percent of the M's starting five in Logan Gilbert, George Kirby and Bryce Miller.

Despite this adversity, the Mariners are nearly back to full health, with Miller the only one left to return and all but certain to be reinstated in the coming days. And with the team playing well as a collective since the trade deadline, thoughts turn to the genuine prospect of qualifying for the playoffs after two agonizing near misses in the past couple of seasons.

The Mariners' conundrum for Game 1 playoff starter

Nothing is guaranteed in the game of baseball, with the Mariners losing three of their last four games serving as a warning, but just make the postseason and this is a team no one will want to face. Which leads us to the question of who will get the start for the M's in Game 1?

Sodo Mojo's Jason Wang wrote about this conundrum just last week, and how the Mariners faced an impossible decision with so many great options. Gilbert is the de facto staff ace, Bryan Woo has been the iron man of the league this season, and Luis Castillo has the most veteran experience.

However, what about this George Kirby guy who didn't make his season debut until May 22, due to dealing with shoulder inflammation?

Unsurprisingly it took some time to get going as he produced a 6.16 ERA in his first six starts off the injured list. Regardless, Kirby has been pitching better than ever in his last nine stats, highlighted by a 2.32 ERA and 2.26 FIP. This includes arguably his better performance of the year in his most recent outing, as he went 7.0 innings of shutout ball on the road in Baltimore, allowing just three hits and no walks while striking out seven in a 1-0 win versus the Orioles.

George Kirby finding his form at the ideal time

As per MLB.com's Ian Quillen, the game represented Kirby's fourth straight quality start and saw his ERA fall below 4.00 for the first time in 2025. Speaking about his recent form afterwards, he said: "I think my offspeed has just been really, really good, especially to lefties. Just hammering that pitch and trying to get ahead. When you get ahead, you can control the game a lot better."

A perusal of Kirby's Baseball Savant page tells you how good things are looking right now, with us finding it intriguing that there's now more balance between the usage of his top three pitches. His four-seamer is still the number one go-to option, but the increased rate of his slider and sinker allude to better confidence in these pitches and by extension makes him more unpredictable to opposing batters.

Kirby is throwing strikes at a higher rate than any other time during his four years in the majors, while also keeping the ball in the yard. The only real concern is that this is also the highest walk rate he's ever had, although he's also allowing his fewest ever hits per-nine innings meaning his WHIP is effectively around the same as last season and is also below his career average.

Overall, the Mariners are working from such a position of strength, that there's actually a possible scenario where Kirby could be left out of the team's three-man rotation altogether, pending qualifying for the playoffs. However, if we look at this situation rationally and logically, he should be in that rotation at the very least, and arguably also be the M's Game 1 starter.