RHP Guillermo Zuñiga
Ok, so I'm cheating a bit. Zuñiga technically debuted back in May. But that was just one scoreless inning, and he hasn't been with the big league club since. After hitting the IL early in the summer, Zuñiga has recovered.
Like Thomas, Zuñiga benefits significantly from the fact that he is indeed part of the 40-man roster. Also like Thomas, his biggest problem is his lack of production in the minors. In 23 games with Memphis, Zuñiga has been awful. He has an ERA of 9.15. Fortunately for him, he has strikeout potential. The Cardinals will want to learn more about how he can contribute in 2024, but the season-long struggles are cause for hesitation. His accessibility and age make him a strong candidate for a call-up.