The Cardinals placed Mikolas on the 10-day injured list before their game Tuesday at Minnesota. Mikolas was supposed to start Wednesday. Cardinals president of baseball operations John Mozeliak said right-hander Daniel Ponce de Leon will take the mound against the Twins instead.
The 31-year-old Mikolas pitched four innings for the Cardinals in an exhibition game last week. He missed spring training because of the injury, but after nearly four more months to rest during the coronavirus shutdown, he ramped up his activity when camp reconvened.
Mikolas wasn't feeling right after his most recent bullpen, Mozeliak said, and an MRI on Monday prompted the procedure. The estimated rehabilitation time is four months. The elbow ligament was deemed fully intact, but pitching through the injury this summer would have put Mikolas at risk for more damage.
Mikolas was 27-18 with a 3.46 ERA in 64 starts for the Cardinals the past two seasons. He tied for the National League lead with 18 wins in 2018. Now the rotation -- which includes Adam Wainwright, Jack Flaherty, Dakota Hudson and Carlos Martinez -- has a hole. Closer Kwang-Hyun Kim was considered as a starter, but the Cardinals decided to keep him in the bullpen.
The 28-year-old Ponce de Leon, who has a 3.29 ERA in 82 major league innings, will get the first crack at filling the spot.
"He's more than earned the opportunity," manager Mike Shildt said.