Rodriguez has been quite solid for the Tigers this year, as he owns a 10-7 record and an ERA of 3.11 over his 21 starts.
But even he does not come without a few risks. He spent a good deal of time on the injured list with a finger rupture. People may not realize it, but finger issues are a big deal for any pitcher. Back in 2008, Adam Wainwright was limited to just 20 starts with a blister on his finger, so the Cardinals certainly understand the risks.
Rodriguez also missed considerable time last season due to a ribcage issue. When healthy, Rodriguez can be a frontline starter, but injuries are an issue that the Cardinals are going to need to be cognizant of.