#6 - Steven Matz signing
Let me start by saying, this ranks a bit higher on my list than you'd probably think because of the potential impact I think this may have on the Cardinals' for the next few years, and I also think it has a chance to move up the ranking even more if their front office doesn't admit the mistake here, or if Steven Matz can turn things around.
The biggest critique of the Cardinals right now is easily their starting pitching, and one major reason why it has taken such a downturn is bad investments like Matz. The Cardinals, like many of the bad signings on this list, love to shop in the "middle free agent market", where the players are not top-end talents, but also won't come cheap either. In my opinion, these are the absolute worst players to target in most cases.
The margin for error is huge for these kinds of signings. If you bring in a Matz to be a number four or five starter, there is a pretty good chance you have someone in your system who could fill his role for $14 million cheaper. But say you have a major hole at the top of your rotatoin like you do right now, it's a lot harder to fill the role of an ace with internal options on the fly. And yet, the Cardinals continue to invest signifcant dollars into back-end starters, relief pitchers, or "role players" for their lineup, when the dollar should actually be going to top-end talent.
So instead of going out and signing Marcus Stroman or Kevin Gausman, the Cardinals signed Steven Matz. Sure, it's a lot less money, but Stroman and Gausman's floors are closer to what Matz's ceiling is as a player, while the ceilings of Stroman and Gausman are players that can lead your rotation in the playoffs.
So far in his Cardinal career, Matz has made 18 starts and 5 relief apperances, posting a 5.42 ERA in 89.2 innigs of work. He's been awful, and while he could end up being an okay rotation piece eventually, his presence in the rotation may actually hinder the Cardinals' beyond just his performance. The Cardinals have Mikolas and Matz both signed through 2025, meaning they'll likely want them both in the rotatoin. That leaves just three spots, and the club also has arms like Matthew Liberatore, Gordon Graceffo, Michael McGreevy, and Zack Thompson ready in Triple-A, not to mention Tink Hence and Cooper Hjerpe eventually as well.
If the Cardinals use this "glut" as a reason not to add an ace in the near future, than the Matz signing will become even worse than it already is.
One way the Cardinals could redeem this signing if by making Matz into a reliever, especially if they need to "clear" rotation spots. Left-handed hitters are slashing just .156/.250/.250 off of Matz this year, which would make him a very good left handed reliver for St. Louis. If you don't think the Cardinals would pay a left-handed reliever $11 million a year, refer back to the Andrew Miller and Brett Cecil signings. The difference here is, Matz could actually be an elite option, unlike those two during their Cardinal careers.