Mikolas, who won 18 games in 2018, was activated Saturday for his first major league start since the 2019 NL Championship Series. He missed last season after flexor tendon surgery, then hurt his pitching shoulder in his first bullpen session of 2021.
Manager Mike Shildt said Saturday that Mikolas was having an MRI after the game.
Mikolas, 32, allowed three hits in four innings, struck out three and walked one, allowing one run Saturday against the Chicago Cubs. He grabbed his right elbow while warming up for the fifth inning and was removed from the game.
The Cardinals recalled right-hander Junior Fernandez from Triple-A Memphis in a corresponding move.
The Associated Press contributed to this report.