Adams is scheduled to have surgery on Friday to address the injury.
"He felt a pop last night," general manager John Mozeliak said. "Early indications are, based on the readings from the MRI, that it's significant."
Adams was placed on the 15-day disabled on Wednesday, and, for the short term, the Cardinals moved catcher Ed Easley, who ranked second in the Pacific Coast League with a .422 on-base percentage, onto the 25-man roster. The Cardinals were not bound to call up a first baseman because they already have Adams' replacement on hand.
Mark Reynolds, an everyday player for most of his nine-year Major League career, now steps in as the team's starting first baseman.
"[Manager] Mike [Matheny] has done a great job of getting me in there as consistently as he can," said Reynolds, who signed a one-year deal with the organization in December. "I feel like I've played a lot more than I thought I would have, so having those at-bats under my belt already will hopefully keep me fresh for what's about to happen."
Reynolds entered the year as one of six players with at least 20 homers in each of the last seven seasons. He has three in 96 at-bats this year, along with six doubles and a .250 batting average. He delivered an RBI single in his first at-bat on Wednesday.
"It's huge that the organization was wise enough to have somebody ready to jump in here," Matheny said. "Not that we were anticipating anything happening to Matt, but to have that depth and have the understanding that that is a spot where we might need some help. ... To have a guy that's had the kind of production Mark has had for a long time, it could be a much tougher spot we're in right now."
The Cardinals have several other players -- Pete Kozma, Yadier Molina, Tony Cruz and Matt Carpenter, among them -- who could step in to play first base in a pinch, too.
The intent to make Reynolds the everyday first baseman played into the organization's decision not to summon Triple-A first baseman Xavier Scruggs. Scruggs, who leads Memphis in home runs (eight) and RBIs (23), would have been limited mostly to pinch-hit duties. With that being the role, the Cardinals instead turned to the Memphis player swinging the hottest bat.
Easley, who did not appear in a game during a three-day callup in April, had a line of .340/.470/.392 this month in Triple-A.
The Cardinals did consider cutting short Jon Jay's rehab assignment with Class A Peoria (Ill.) but will instead leave him there for another two games. Jay, who is coming off a left wrist injury, went 0-for-4 for Peoria on Tuesday. He is expected to be activated from the DL on Friday.