It's the same injury that put Cardinals infielder/outfielder Matt Carpenter on the 15-day disabled list on Wednesday.
Carpenter exited Tuesday night's game against the Padres in the first inning with pain he described as a "knife in [his] rib" after hitting a double down the left-field line.
Carpenter was initially hopeful that the injury wouldn't put him on the DL, but an MRI exam on Wednesday showed otherwise.
"I think he's got a really high pain tolerance, to where it didn't seem like that big a deal, but it is," Matheny said Wednesday.
Carpenter said the muscle was more sore Wednesday morning than it was Tuesday night. However, Carpenter added that he has more mobility than most others who endure the same injury and hopes to make a speedy recovery. He said he'll still continue to lift weights regularly.
"I've always been a guy who always goes after it in a hard fashion," Carpenter said. "And they've tried to pull the reins back on me a little bit to make sure that I take care of it and not let it be something that lingers all year."
The Cardinals called up catcher Steven Hill from Triple-A Memphis to replace Carpenter on the 25-man roster. Carpenter is the sixth Cardinal to be placed on the DL since May 16.
Matt Adams is expected to get the majority of starts at first base until Carpenter and/or Allen Craig (left hamstring strain) return from the DL.
"It's just part of the gig," Matheny said. "I think we knew that going in...we're not immune, and that's just part of it."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.