Here’s Outside-The-Box Red Sox Trade Idea If Mariners Want Pete Alonso

The Seattle Mariners might have their eye on Pete Alonso, and if the M’s can land the slugger, the Red Sox could help make it happen.

That would come with its own benefit for Boston, of course, if the Mariners are willing to part with pitcher Luis Castillo to help make Alonso happen.

The Boston Globe’s Alex Speier reported late Sunday night that the Mariners have “expressed willingness to listen to offers” for Castillo. On Monday morning, MLB Network’s Jon Morosi also noted Seattle might look to move Castillo, especially if it meant clearing up money for Alonso.

“I think you could see some West Coast teams, like the Mariners if they can clear some money. That’s the big question,” Morosi said Monday morning on MLB Network. “Can Seattle clear enough money with potentially moving Luis Castillo to afford Pete Alonso?”

Castillo is about to enter the third year of a five-year, $108 million deal. That deal comes with a $21.6 million annual clip against the luxury tax, and he’s owed $22.75 million cash over the next three seasons with a vesting option for 2028. According to Spotrac, the vesting option (for $25 million) vests if he logs 180 innings in 2027. There’s also protection in there against injury; if Castillo misses at least 130 days in any of the next three seasons because of an UCL injury, it becomes a $5 million team option.

As the Red Sox continue to turn over rocks in search of starting pitching, Castillo is at least worth consideration. While he carries a hefty price tag, it’s not a long commitment that would come with signing someone like Corbin Burnes. Depending on how much money Seattle wants to clear, the Red Sox could work off the major league roster to acquire Castillo and not have to dip too much into the prospect pool.

The big question: Is he worth it? Castillo turns 32 on Thursday. He has logged more than 1,200 innings in his big league career.

He’s also coming off a bit of a down season in which he went 11-12 with a 3.64 ERA, his highest mark since 2021. What’s just as concerning is his strikeout rate dropped — from 10 punchouts per nine innings to nine — and his 3.91 FIP, his highest since 2018, might also suggest his season was even worse than the basic numbers suggest.

However, if the Red Sox believe that’s a relative blip on the radar, his body of work is impressive. Castillo has twice led the league in starts, logging at least 30 starts in five of his eight seasons, including 30 in 2024. He’s a workhorse. Even if he can be the pitcher he was in 2024 for at least a couple of more seasons, it will represent an upgrade on what the Red Sox rolled out last season.

And while the contract might be slightly uncomfortable given his age and recent dip in production, anything close to his career average probably is valuable in this market. The Red Sox should still try to sign a top-of-the-rotation arm like Max Fried. But given the kinds of money being thrown around for the likes of Yusei Kikuchi and Luis Severino, a No. 2 or 3 like Castillo at basically the same money looks even better.

The Red Sox would be far better off acquiring someone like Garrett Crochet, who is a legitimate game-changing talent, especially from the left side. If the Red Sox can’t land the Chicago White Sox stud, though, they’ll again need to pivot, and Castillo is the sort of arm they should consider.