Cardinals general manager John Mozeliak told reporters that Wainwright experienced soreness, similar to "tennis elbow," after throwing seven shutout innings in Tuesday's victory over the Tampa Bay Rays.
Wainwright, who acknowledged after the 1-0 win that he was pitching with "average stuff," returned Wednesday to St. Louis, where he underwent the MRI.
Mozeliak did not disclose whether Wainwright would miss his next scheduled start.
"We know the importance of him to our rotation and the importance of making sure he's in the rotation," Mozeliak told reporters, according to the St. Louis Post-Dispatch. "We want to be extra cognizant of what he needs to stay in the rotation and be healthy from here. We are going to be cautious with this."
Wainwright, 32, is 9-3 with a 2.15 ERA in 14 starts this season for the defending National League champion Cardinals. He missed the entire 2011 season after needing Tommy John surgery to repair a torn ligament in his elbow.