Cardinals general manager John Mozeliak said Wainwright could have a "significant injury."
If that's the case, Tommy John surgery could be an option, which would end the right-hander's 2011 season before it begins.
Wainwright returned to St. Louis for tests from the Jupiter, Fla., spring training camp after apparently experiencing pain while throwing Monday.
The 29-year-old Georgia native was 20-11 with a 2.42 earned run average in 33 starts last season, finishing second to Philadelphia's Roy Halladay in Cy Young voting.
As a reliever, Wainwright helped the Cardinals win the World Series in 2007, then was moved into the starting rotation.