The Cardinals activated Gregerson off the 10-day injured list ahead of Saturday's game against the Chicago Cubs.
Gregerson was placed on the injured list three times in 2018, with the shoulder injury in July ending his season. He also missed two months earlier in 2018 with the same injury.
He has pitched just 12 2/3 innings since signing a two-year contract with the Cardinals in the 2018 offseason. The 34-year-old has a career ERA of 3.10 while playing for the San Diego Padres, Oakland Athletics, Houston Astros and Cardinals.
In a corresponding move, outfielder Tyler O'Neill was optioned to Triple-A Memphis. O'Neill was hitting .263 with one home run and 4 RBIs in 21 games this season.
The Associated Press contributed to this report.