Mariners' Luis Castillo takes on Red Sox after they were too cheap to trade for him

The Seattle Mariners will have one of their best pitchers on the mound when they go for a series win over the Boston Red Sox on Wednesday afternoon at T-Mobile Park. And given the circumstances, Luis Castillo might be a little extra motivated to go out there and shove.

In case anyone has forgotten, the Red Sox made a serious attempt to trade for Castillo over the winter. As detailed by Mark Feinsand of MLB.com last December, the veteran righty was among Boston's targets for a subsequent rotation addition following the blockbuster acquisition of Garrett Crochet. But the Mariners wanted Triston Casas, and the Red Sox were unwilling to give him up unless Masataka Yoshida and the $55.8 million left on his contract also went to Seattle.

The Red Sox eventually pivoted to signing Walker Buehler, seemingly closing the book on the story of how they tried to pluck a three-time All-Star out of the Pacific Northwest. However, it turns out there's more to it.

Luis Castillo wanted the Red Sox to pony up more money to complete an offseason trade

Jared Carrabis went on the Marine Layer Podcast on Tuesday and dished some details of the Mariners' negotiations with the Red Sox that hadn't previously been public. The whole segment is worth a listen:

The key information concerns Castillo's position that his contract be restructured to account for Massachusetts state taxes, at which the Red Sox balked.

This alone could have killed whatever agreement the Mariners and Red Sox may have come to. The five-year, $108 million contract that Castillo signed with Seattle in 2022 includes full no-trade protection for 2023-25, so he could have blocked the deal if he didn't get what he wanted from the Red Sox.

That he took such a position on his contract is wholly understandable. Massachusetts taxes income at a base rate of five percent, with an additional four percent surtax on income exceeding $1,083,150. That would have affected Castillo, who is making $24.15 million in 2025.

How athletes pay state taxes is wonky, but most states (including Massachusetts) have a "jock tax" that requires athletes who perform in the state to file a state income tax return and pay a portion of their earnings to the state. Hypothetically, Castillo would have owed roughly $1 million in taxes to Massachusetts even if he'd made just half his starts in Boston this year.

Which brings us to how Washington handles income taxes: There aren't any, as the state only taxes capital gains. It's a nice perk for all Seattle athletes, Castillo included.

Since this is all a lot to process, let's all take a break with Castillo throwing a nasty sinker:

In any case, the Red Sox not trading for Castillo largely comes down to them being too frugal to do so. And several months later, the joke is on them.

The 32-year-old Castillo continues to be an ageless wonder, as his first 14 starts have yielded a 3.29 ERA over 79.1 innings. The Red Sox, meanwhile, have thus far gotten a 5.95 ERA out of Buehler, who was ruthlessly shellacked by the Mariners on Tuesday. They've also lost Casas for the year, the fallout of which played a huge role in the breakdown of the breakdown of their relationship with Rafael Devers. For his part, Yoshida is still recovering from offseason shoulder surgery.

In retrospect, the negotiations Seattle and Boston had about Castillo during the winter look like a remarkable sliding doors moment. The Mariners should be grateful that Castillo ended up staying, and that much more so if he's the reason they take two of three from Boston today.