The Mariners will be in trouble if Luis Castillo doesn't shake off his home run problem

No one's under any illusion about Luis Castillo still being the same pitcher who the Seattle Mariners traded for three years ago. He's no longer the team's ace; no longer the player who was selected to three All-Star Games between 2019 and 2023.

However, Castillo still has value in the Mariners rotation — you're in a pretty damn good situation as a team if he's (at least unofficially) your third option behind Logan Gilbert and Bryan Woo, and probably soon fourth with George Kirby slowly rounding into form. In this respect, he's been effective enough so far in 2025, but an issue has started to rear its ugly head in recent starts for the righty.

In his first 12 starts, Castillo only allowed four home runs. In his last four outings, he's given up eight, which is troubling to say the least. The question is: Will this continue to be a problem, or is it something than can be mitigated moving forward?

An understandable overall regression for Luis Castillo with the Mariners

In a general sense you can put this down to some inevitable regression for a pitcher who's now 32 years old. To an extent this is shown by a stat line that includes a 3.69 ERA, 4.20 FIP and 1.273 WHIP, which would all project to be the second-worst of his nine seasons in the majors.

More specifically, the recent issues with allowing more home runs is linked to a decline in Castillo's average velocity over the course of the last few years, with a particular focus on his heater. As per Baseball Savant, five of the eight homers given up in his past four outings have been against fastballs.

It's also important to note that all eight home runs have been in the strike zone, alluding to Castillo conceivably tricking himself into believing he can be more effective in there than he actually is. As per FanGraphs, this is backed up by the righty living in the strike zone more often than before.

The thinking here is that the former Cincinnati Red is relying too much on his fastballs and hitters are sitting dead red and lapping it up. In fact at 45.9 percent of the time he's relying on his four-seamer, more than at any point in his Major League career since his rookie year in 2017, when he used his heater 46.6 percent of the time.

Decreased velocity + more fastballs = insanity

Making this more interesting and confounding, is the aforementioned reference to how Castillo's average velocity has declined in recent seasons. In 2022 he averaged 97.1 mph on his fastball, which then dipped to 96.3 and 95.6 in successive seasons and is now down to 95.1 so far in 2025.

Another way to consider the negative impact of Castillo's declining velocity — both with the four-seamer specifically and his pitching in general — is his strikeout rate. When he first joined the Mariners in 2022, it was 28.9 percent. But this year, he has dipped below 20 percent for the first time and is currently projected for a career-worst 19.7 percent.

In terms of saving himself and the Mariners, it's imperative for Castillo to move away from using his heater so much, with 2022 serving as the best example of a more even spread of his weapons and a potential model to follow. This was the only season all four of his main pitches were used at least 20 percent of the time.

Regardless, if he doesn't adjust and come up with a new approach soon, then you fear the recent increase in the number of home run blasts from opponents will continue.