"No sense in trying to pitch him at this point," St. Louis manager Mike Matheny said Wednesday.
Gregerson signed an $11 million, two-year contract. The 33-year-old right-hander has appeared in one exhibition game, striking out two in a hitless inning on Saturday.
Not a traditional high-velocity closer, Gregerson relies on motion and sink.
"Some of the ugliest swings we've seen all spring," Matheny said after Saturday's outing. "Gregerson walked out there and just had a different look to it. The break was different. His movement, his location. They were different at-bats. You see guys taking pitches, they are jumping out of the way and it's called a strike. That's just telling you it's not like everybody else."
Gregerson first began feeling the tightness in the days that followed that appearance. He originally was scheduled to throw again on Monday but instead met with the trainers.
"I think he's going to be held back a little bit," Matheny said. "From what I heard, it wasn't one of those ones that were like, whoa!" Matheny said.
Gregerson is 35-36 with a 3.02 ERA in nine big league seasons. He saved 31 games for the Houston Astros in 2015 and 15 more the following season, before Ken Giles took over as closer last season for a team that ended up winning its first World Series title.