The Mariners have All-Star Mitch Haniger in right field, but the rest of their starting outfield is Guillermo Heredia and Denard Span. While they’ve played decent, that’s not an outfield that will win a team a World Series. Ben Gamel has been a capable fourth outfielder, but the Mariners could use another slugging outfielder. Castellanos could transition over to left field, allowing Span to be more of a fourth outfielder/platoon option.
The Mariners also need some help at first base, although Robinson Cano‘s return from suspension may involve him platooning at first with Ryon Healy. Moving Castellanos midseason is not a great move, but Seattle could transition him to first base during the offseason as well, if they chose to.
It’s not a perfect fit, but Castellanos would give the M’s some more oomph in their lineup. A top six of Dee Gordon, Jean Segura, Nicholas Castellanos, Nelson Cruz, Kyle Seager and Mitch Haniger would be elite.
They don’t have a ton of prospects that would be of interest to Seattle, since Top-100 prospect Kyle Lewis is likely off the table. However, Daniel Vogelbach is a power-hitting first base prospect, and could be of interest to the Tigers. Miguel Cabrera will certainly move to DH next season, leaving first base as a position of need.