Mariners saw the warning signs that culminated in Luis Castillo’s White Sox demotion

When Seattle traded Luis Castillo to the Chicago White Sox at the trade deadline, the hope in Chicago was pretty obvious. They were betting on a fresh start helping Castillo return to the pitcher he once was.

Nearly two months later, however, things have gone from bad to worse, and Chicago has moved Castillo to the bullpen after seven brutal appearances since the trade. The White Sox acquired Castillo to stabilize a rotation that desperately needed help. Instead, the 33-year-old right-hander wasn't able to provide them with even five consistent innings.

On Thursday, according to Jesse Rogers of ESPN, White Sox manager Will Venable confirmed that Castillo's next opportunity will come out of the bullpen, with rookie David Sandlin taking his spot in the rotation.

Ahead of the demotion, general manager Chris Getz opened up about the changes, as reported on MLB.com by Brian Dulik.

“The quality of pitches have definitely gone in the wrong direction,” Getz admitted. “We’ve got to solve it. We do. We’re continuing to look at some of the trends and the changes that have happened [since he left the Mariners]. It has been surprising.”

To be fair, Castillo's pitch quality has gone in the wrong direction since the trade, but...

What did Chicago expect from Luis Castillo?

Before the trade, Castillo had already put together the worst season of his Mariners career. He had a 5.06 ERA when Seattle traded him, and now, his strikeout rate has fallen, his home-run rate has climbed, and his overall results have deteriorated. While Castillo's ERA rose from 3.34 in 2023 to 3.64 in 2024, it settled at a 3.54 in 2025. The 2026 season was the first time his decline turned into something to be concerned about.

Through 20 games with Seattle, including 17 starts, he was 3-9 with a 5.06 ERA and 86 strikeouts over 99 innings. With those numbers, it's not like the Mariners were trading away the 2023 version of Luis Castillo.

There were warning signs, and Seattle decided it was time to move on. Meanwhile, Chicago saw the same pitcher and hoped for a return to form. Instead, they are seeing the down side of that decision, and what many Mariners fans saw coming for some time.

But, in Chicago's defense, there was some reason for optimism. Castillo had been a productive starter in previous seasons. Then on August 12th, Castillo gave Chicago a glimpse of that pitcher when he threw seven shutout innings against Cincinnati, allowing just one hit while striking out 10. Unfortunately, that version of Castillo didn't stick around, and things went downhill, fast.

Castillo has made seven appearances for Chicago, including six starts, and posted a 7.76 ERA over 29 innings. He's allowed 41 hits, 25 earned runs, and 11 walks while striking out 25. His WHIP sits at 1.79.

Now, as the White Sox sit just behind the Cleveland Guardians in the race for the American League Central title, Chicago has decided they can no longer wait for Castillo to return to form in time to help them out this season in the way they initially hoped.

The Luis Castillo trade is already netting small wins for Mariners

As the White Sox move toward postseason play, the Mariners' season seems all but over. And while it's too early to declare the trade a complete win, it is important for Seattle to acknowledge the small wins while they still can this season.

Seattle didn't just move on from a starter whose performance was trending in the wrong direction. They also acquired reliever Seranthony Domínguez, outfielder Nolan Jones, and catching prospect Boston Smith while offloading the remainder of Castillo's hefty contract.

Maybe Castillo turns things around, but for now Mariners fans who watched his struggles in Seattle and wondered why Chicago was willing to take on the risk, they can't help but say "We told you so."

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