#11 - The Luke Gregerson for Khalil Green trade
For much of the 2000s and 2010s, it felt like the Cardinals could never land on a long-term answer at shortstop. With this being one of the most important positions in baseball, it makes sense why St. Louis took so many swings at the position. Trading for former esteemed prospect Khalil Green was a big swing, and it turned out poorly for the club.
Greene only played in 77 games for St. Louis before severe anxiety forced him out of baseball entirely. We know a lot more about the importance of mental health in today's world, but his struggles were not something widely talked about back in 2009. This set the Cardinals back at shortstop once again.
On top of that, St. Louis gave up one of the most reliable relievers of the 2010s, Luke Gregerson, in a package that also included Mark Worrell.
Gregerson went on to pitch eight seasons before returning to St. Louis, posting a 3.02 ERA in 599 innings for the Padres, Athletics, and Astros. That kind of production over an eight-year period from a reliever is pretty rare, and the Cardinals missed out on that production dearly. In his return to St. Louis, Gregerson only threw 18.1 innings with a 7.36 ERA.