John King should be a potential roster casualty to make space for McGreevy, but he won't be
I actually really like John King, but if the Cardinals are going to make room for McGreevy in their rotation, it will require them moving Matz to the bullpen, and that would also mean that one of JoJo Romero, Matthew Liberatore, or John King must go down to Memphis.
In theory, sending Liberatore to Memphis to continue pitching as a starter would make the most sense. He's been great in his audition for that role in camp, but the Cardinals have already stated that he will begin the year in the bullpen instead (with the chance to still start at some point). Obviously the Cardinals are not going to send down JoJo Romero, so the clear candidate here would be King.
King was valuable for the Cardinals last year, posting a 2.85 ERA in 56 appearances, albeit with a 3.73 FIP. As a groundball merchant, King is reliant a bit on batted ball luck going his way, and last year he saw a career-best .279 BABIP allowed while he was on the mound. I do think he will be a part of the Cardinals bullpen much of this year, but I don't think he should be a lock for that group if it's the difference between McGreevy starting in the rotation on Opening Day or not.
In seven appearances this spring, King has a 5.40 ERA, allowing 11 hits in just 6.2 innings of work and walking two batters. Again, I'm not all that worried about King, but it has not looked great so far.
The Cardinals won't be doing this though. King will be on the roster and Matz will be in the rotation, with McGreevy down in Memphis. You could tell me that it makes sense because if they want Matz as a rotation option, then putting him in the bullpen now goes against that. But I would counter with Mozeliak's own logic: How could Liberatore still be a rotation option if he's going to start the year in the bullpen? Apparently they don't think that would be an issue for him. Well, then it should be even less of an issue for Matz.