LHP Mason Montgomery, Tampa Bay Rays
Alright, maybe dealing for another left-handed pitching prospect from the Rays would be a mistake, but I can see this being a good match for both teams.
The Rays are one of the favorites this year, and with how often they've come close in recent years, they seem to be motivated to be more aggressive this year. Their offense could use another bat or two, and another starter wouldn't hurt them either with the injuries they've experienced recently.
A package with Montgomery in it and a few other strong pieces could be enough to get both a young bat and one of the Cardinals starters. With the way the Rays develop starting pitching, it's hard to imagine they wouldn't raise up more arms to replace him, fast.