Garrett Whitlock, RHP (and Red Sox UTIL Rob Refsnyder)
Even after a tough 2023 debut as a starter, this list still can't live on without acknowledging that the Yankees weren't willing to wait out Garrett Whitlock's Tommy John rehab (or had no idea what his peak could look like) and opted to protect erratic non-entity Brooks Kriske instead.
Somehow, the Red Sox managed to catch one of Whitlock's Instagram throwing sessions (was Brian Cashman not following him on IG?) and decided he was worth a selection. Survey says ... yup! Whitlock was an ace-level bullpen contributor and innings-eater in 2021, posting 2.9 WAR and a 1.96 ERA in 73.1 innings with the help of a magic fastball. Since then, he's signed a contract extension and been jerked back and forth from the rotation to the bullpen and back again, something we can guarantee you the Yankees wouldn't have done *cough, Joba, cough*.
Add in honorable mention -- and fellow Red Sox -- Rob Refsnyder here. Sure, it took Ref a long time to figure it out -- well after the "Give Rob the Job" days in the Bronx -- but he's become a very effective offensive piece against left-handers, ultimately unlocking something and hitting .307 with a .384 OBP and .881 OPS (142 OPS+) for the 2022 Sox.
Sure, it took a while for Refsnyder to find what made him so special as a prospect, but he did hit .302 with an .859 OPS in his first 47 plate appearances as a Yankee in 2015. Just a year and a half later, New York felt the need to trade him ... and not in a blockbuster, but in a random midseason 40-man swap for slugging first baseman Ryan McBroom (raise your hand if he affected your life in any way). Probably could've waited and uncovered what the Sox eventually did.