St. Louis Cardinals right-hander Miles Mikolas exited his start on Saturday against the Chicago Cubs with an injury the team described as "right forearm tightness," according to Katie Woo of The Athletic. He called for a trainer before the beginning of the fifth inning. 

Mikolas, 32, was making his first regular season appearance since September 22, 2019. He missed the entirety of the 2020 campaign after undergoing surgery to repair a flexor tendon. He was then sidelined for most of the first two months of this season because of a sore shoulder.

The Cardinals placed Mikolas on the 10-day injured list on Sunday following the injury.

Mikolas had held the Cubs to a run on three hits and a walk in his four frames. His average fastball velocity on the night was 89.8 mph, well below his 2019 average of 93.7 mph. 

Mikolas made 64 starts over the 2018-19 seasons with the Cardinals, compiling a 3.46 ERA (116 ERA+) and a 4.75 strikeout-to-walk ratio. He has two additional seasons remaining on his contract, each worth $17 million.

The Cardinals are already without a number of key pitchers. Fellow starter Dakota Hudson is out for the season after undergoing Tommy John surgery last September, while relievers Jordan Hicks and Andrew Miller have been on the shelf for nearly a month apiece. The Cardinals are also without starting shortstop Paul DeJong and outfielder Tyler O'Neill.

A prolonged absence from Mikolas would dash the Cardinals' plans of using a six-pitcher rotation for the immediate future. "The guys have pitched well and the starters have given us an opportunity to win games most every night," manager Mike Shildt explained to the St. Louis Post-Dispatch. "It's been a nice benefit. It doesn't feel like we have to go to a five-man (rotation)."

The Cardinals entered Saturday night with the 11th best rotation ERA in Major League Baseball.