Option 1 - Zack Thompson
While I believe Zack Thompson is better suited for a long relief role in the late innings as a high-leverage left-hander, he did show significant promise as a backend rotation option toward the end of the 2023 season. In nine games started, Thompson recorded a 4.57 ERA in 45.1 innings pitched. He was slightly better in relief, but not enough for it to be noticeable. For a backend starter, this is more than serviceable and significantly better than the Cardinals were able to get from the combination of Adam Wainwright and Jake Woodford last season.
Thompson should move back to the bullpen, where he's better suited, for a Postseason run but for the start of the season or filling in for an injured starter he's proven that he's good enough. Fans often overvalue how productive a team's backend starter should be.
The Cardinals by no means will have a super-rotation with Thompson at the back like the Dodgers or Mariners who have promising young talents such as Bobby Miller and Bryan Woo at the backend, but a super-rotation is not needed to contend in the playoffs. Thompson is better than the fifth or sixth starters of most Major League clubs, so there should be no concerns about letting him start a game or two.
Option 2 - Matthew Liberatore
Matthew Liberatore has had as rocky of a start to his MLB career as a starting pitcher could have. Through 31 appearances and 18 starts, Liberatore has recorded a 5.51 ERA and 4.54 FIP, not nearly the numbers fans expected from a once top-100 pitching prospect. It's still too early to say whether or not Liberatore should be considered a bust, but time is running out for him to prove himself. It's been reported that the Cardinals may look to move him to a bullpen role moving forward, but one last chance in the starting rotation may help Libby turn the page.
His command and velocity have wavered in late innings, but with the additions of Gibson and Lynn in the backend of the rotation to help eat innings, letting Liberatore struggle for a bit before finding his footing as a potential starter of the future for the Cardinals could work to their advantage. After all, if Liberatore has a few shorter outings in the early season, the bullpen won't be as overworked as it was last year so they can fill the remainder of innings.