Max Scherzer, 2014-15 offseason
Throwing it back a bit here, but the Yankees found themselves embarrassed into committing to a free agent splurge in 2013-14, following yet another Red Sox title.
That four-man class consisted of Jacoby Ellsbury (holy hell, no!), Brian McCann (predictable regression), Carlos Beltrán (fine, but was inspired to invent Astros' cheating scheme while in New York), and Masahiro Tanaka (beloved No. 3 starter).
If those non-Cano splurges had all worked, the Yankees might not be in need of further reinforcements. But with Tanaka's elbow barking, they had a chance to make it all worthwhile by paying Max Scherzer -- who cost nothing but money -- to really take over the rotation.
The 2015 Yankees somehow made the postseason. scratching and clawing to host (and lose) a Wild Card Game against the Astros. Imagine if they'd matched Scherzer's reasonable deal (seven years, $210 million) in Washington, during which he never finished with a full-season ERA over 3.00 (2020 is fake), won back-to-back Cy Youngs, and finished in the top three two other times?
They didn't, though. They continued to struggle to find a rotation topper until Gerrit Cole arrived. Oops!