The Seattle Mariners may have their sights set on Pete Alonso, and if they manage to land the slugger, the Boston Red Sox could play a role in making it happen. Naturally, this would come with a potential benefit for Boston if the Mariners are willing to part with pitcher Luis Castillo to facilitate an Alonso acquisition.
According to Alex Speier of The Boston Globe, the Mariners have shown a “willingness to listen to offers” for Castillo. MLB Network’s Jon Morosi echoed this sentiment, noting Seattle might consider moving Castillo to clear payroll space for Alonso.
“I think you could see some West Coast teams, like the Mariners, clearing money. That’s the big question,” Morosi said. “Can Seattle move enough salary, potentially by trading Luis Castillo, to afford Pete Alonso?”
Castillo is entering the third season of a five-year, $108 million contract, which carries a $21.6 million annual luxury tax hit. He’s owed $22.75 million annually through 2027, with a $25 million vesting option for 2028 if he logs 180 innings in 2027. The deal also includes an injury clause: if Castillo misses 130+ days in any of the next three seasons due to a UCL injury, the option becomes a $5 million team option.
For the Red Sox, Castillo could be an intriguing option as they continue searching for starting pitching. While his contract is substantial, it’s not as lengthy a commitment as signing someone like Corbin Burnes. Depending on how much salary Seattle wants to shed, Boston could potentially acquire Castillo by trading from their major league roster rather than depleting their farm system.
But is Castillo worth it? He turns 32 this week and has pitched over 1,200 innings in his MLB career. His 2024 season, however, was slightly underwhelming—he went 11-12 with a 3.64 ERA, his highest since 2021. His strikeout rate dipped to 9.0 per nine innings (down from 10.0), and his 3.91 FIP, the highest since 2018, suggests his performance may have been even shakier than the surface stats indicate.
Still, Castillo’s overall track record is strong. He’s led the league in starts twice, making at least 30 starts in five of his eight seasons, including 30 in 2024. If Boston believes his 2024 struggles were a minor hiccup, his durability and past performance make him an upgrade over their current rotation.
While Castillo’s contract may seem a bit uncomfortable due to his age and recent decline, his career averages remain valuable in today’s market. Ideally, the Red Sox would pursue a top-tier arm like Max Fried. But given the soaring prices for pitchers like Yusei Kikuchi and Luis Severino, acquiring a No. 2 or No. 3 starter like Castillo at a similar cost might be a savvy move.
That said, the Red Sox could be better served targeting a younger, high-upside arm like Garrett Crochet from the White Sox. If Boston fails to land Crochet, Castillo is a reasonable alternative to bolster their rotation.
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