#9 - Andrew Miller signing
From 2013-2017, Andrew Miller was one of the best relievers in all of baseball. After struggling a bit for Cleveland in 2017, Miller was a free agent, and the Cardinals brought him in on a two-year, $25 million deal with a $12 million vesting option for the 2021 season.
During his Cardinals' tenure, Miller put up 4.34 ERA over 103.2 innings of work, never living up to the contract he signed, but was still a helpful lefty to have out of their bullpen. During the stortend 2020 season, Miller looked really good with a 2.77 ERA, but that was over just 16 games of work.
While Miller was certainly a bad signing at that value, I won't hate on the move too much since the idea of it made a ton of sense at the time, and the amount of dollars per year, about $12 million, wasn't enough to tie the club's hands if they wanted to make a significant move. But the move certainty did not pay off,
In general, as we'll see on this list, the Cardinals' investments in relievers have not really panned out, but this one was defensible in the moment and wasn't as disastrously as other moves they have made.