Mariners could get another crack at trading for Red Sox slugger on shaky ground

The Seattle Mariners aren't being cute about how they want to fill their hole at first base. Josh Naylor was the guy they had at the end of 2025, and Josh Naylor is the guy they want to be there in 2026 and beyond. It's not rocket surgery.

And yet, it doesn't take much to imagine dominoes falling in such a way that the Mariners are forced to take another look at Boston Red Sox slugger Triston Casas.

The Mariners had to seriously consider Casas last winter, precisely because the Red Sox asked them to in trade talks centered on starting pitching. Boston wanted Bryce Miller or Bryan Woo for Casas, to which the Mariners pretty much said "thanks, but no thanks." It sure looked like the Mariners avoided a worst-case scenario as the 2025 season played out. Though Miller endured a lost year, Woo broke out as an All-Star and Casas' season ended in May when he ruptured his left patellar tendon.

Mariners could get another crack at Triston Casas if the Red Sox make him available

There were rumblings in July that the Red Sox weren't convinced Casas would be able to return to being a lineup force, and his future in Boston has only gotten less certain. Casas has been saying some cryptic stuff on Instagram, and Bob Nightengale of USA Today has heard from rival GMs that the Red Sox may be the perfect landing spot for Pete Alonso.

The dots are easy to connect. If Casas isn't already a trade candidate, he surely will be if the Red Sox reel in Alonso to take his place at the cold corner. And if the Mariners miss out on Naylor — which doesn't seem probable, but is possible if his market extends into the nine-figure range — there would be more dots to connect between Boston and Seattle.

The difference this time around is that Casas would be a buy-low target for the Mariners. He is only 25 and under club control through 2028, but he was only batting .182/.277/.303 prior to his knee injury, which also fits within a broader narrative of him being prone to injuries.

The appeal for the Mariners would be that he had a 127 OPS+ across 2023 and 2024, with a 162-game average of 31 home runs. In the former season, he actually ranked 21st overall with his .370 xwOBA. There's swing-and-miss in his game, but also zone discipline and real power.

If the Red Sox still wanted to extract a pitcher in a Casas trade, the Mariners could oblige... just not in the way they would have had to last winter. It would have to be a buy-low trade for both clubs, so something like this could theoretically work:

The Red Sox were involved with Luis Castillo last winter, only for talks to fall through in part because of the righty's desire to have his contract restructured. As Castillo's no-trade clause has since expired, he wouldn't have leverage to say no to a deal this time around.

Castillo is owed $24.15 million in each of the next two seasons, compared to $12.5 million per year for Jordan Hicks. The Mariners would therefore get a nice bit of savings (and a wild card bullpen arm with big-time stuff) in addition to Casas, while the Red Sox would be swapping out two headaches for near-term certainty in their rotation after Garrett Crochet.

For now, this is purely fantasy baseball stuff. And it will stay that way if the Mariners can keep Naylor, which is in that sweet spot as a move that everyone wants and which most actually expect. But if they do end up needing to pivot, there's little reason to think Casas won't be there for them.