Luis Castillo's dying fastball should seal his fate in Mariners' rotation

Last season was a special one for the Seattle Mariners rotation, and one of the most intriguing aspects was that Luis Castillo was the least productive member of the preferred starting five. He ranked as the worst in bWAR, ERA, ERA+, FIP and WHIP.

This was surprising given Castillo was the Opening Day starting entering the 2024 season, had the most experience, and was coming off the third All-Star selection of his career a year earlier. Some wondered if this alluded to just an inevitable beginning of a decline which usually happens once players reach their 30s.

As a result of all this, the 32-year-old lost his role as the M's Opening Day starter this season to Logan Gilbert. However, give him his due as he didn't sulk, and for the majority of this season has proven to be the most reliable starter in the rotation behind Bryan Woo, as evidenced by the second-most innings pitched and second-highest bWAR.

August has been tough for Luis Castillo

Unfortunately, things have begun to go wrong for Castillo since we flipped the calendar to August. In five starts he's averaging less than 5.0 innings per outing and has produced a 6.66 ERA, 6.70 FIP, .324 opposing batting average and 1.64 WHIP.

At the heart of all this is a drop in Castillo's average fastball velocity, which has been on a steady decline since he joined the Mariners, from 97.1 mph in 2022 to 95.0 mph so far this season. Most concerning is that the average velocity has dropped particularly in August, to represent its lowest point of the 2025 campaign:

With a decreased average velocity in Castillo's fastball, this sure seems to make for little margin for error — something which he and the Mariners found out big time in a disastrous opening inning of his most recent start on Tuesday night at home against the San Diego Padres:

Castillo talked about the misplaced fastball in the aftermath of a tough 7-6 loss for the Mariners. As per Ryan Divish of The Seattle Times, speaking through an interpreter he said: "Incredible. I wouldn’t say it was a bad pitch. But I definitely could’ve thrown that pitch a little higher up in the zone. Being up with two strikes and throwing that pitch and him being able to connect with it, you have give him all the credit on that."

Given this recent downturn in Castillo's overall form and average fastball velocity, some have wondered if he has suffered some type of injury. However, he was quick to deny this while also maintaining optimism that he will come through this funk, as he said: "... I feel healthy. My arm feels good, my body feels good. When I go up there, I try to throw as hard as I can. So to me, it’s surprise. Hopefully it’s just part of those bad moments you’re going to have in a season. And hopefully, once we get out of this slump, we start seeing an increase in velocity."

Of course M's fans would prefer Castillo's optimism to be justified, but we have to at least consider the possibility he's in denial and won't be able to turn things back around between now and the end of the regular season. If this is the case, at least he will make things easier for the team when it comes to an important decision pending making the playoffs.

Odd man out in the Mariners' postseason rotation?

There has been plenty of discussion about who the Mariners' three-man rotation should be in the postseason. Woo is unquestionably included, and it clearly has to then be Gilbert and George Kirby over Castillo.

In fact it's gotten so tough for the Dominican Republic native, that for a Game 4 you can now make a case to even pick Bryce Miller over him. As much as Miller has admittedly not pitched well since coming off the injured list, as per FanGraphs at least the velocity is there in comparison to his teammate.

There was plenty of speculation prior to the 2025 season about the Mariners considering trading Castillo, but he always said he never wanted to leave. He was also in a position of strength with this given the full no-trade clause in place on his contact, but that ends after this year.

With all the injury issues which plagued the Mariners rotation earlier in the season, keeping the righty around worked out to everyone's advantage. However, you do have to wonder if his time in Seattle is close to concluding, especially if he can't find a way to rediscover some heat in a fastball which is compromising his productivity and the team.