McClellan, the most versatile pitcher on the Cardinals' staff, left Thursday's 7-5 loss to the San Francisco Giants after throwing a couple of pitches to Emmanuel Burriss in the bottom of the seventh inning.
"He felt some discomfort," St. Louis manager Mike Matheny said. "I don't want to jump to anything just yet. We'll let the doctors take a look at him."
McClellan will fly to St. Louis while the rest of his teammates open a three-game series against the Los Angeles Dodgers on Friday night.
"I've had two surgeries and been through everything," McClellan said. "The pain is somewhat similiar to when I did it in the Minors [in 2006], but that was on one violent pitch. I'm hoping this isn't like that."
"Like that" would mean ligament damage and a lengthy stay on the disabled list.
"I thought it was something that we deal with on a daily basis in the bullpen," McClellan said. "It's been sore for a couple of weeks, but it got to the breaking point where I knew I couldn't go on."
McClellan has been a relief pitcher much of his professional career, but has spent portions of several seasons in the starting rotation.
He took over when Adam Wainwright was forced to miss last year following Tommy John surgery. He returned to the bullpen and was used extensively during August. He was later shut down for the season and did not appear in the postseason.
"I've had three days off since the last time I threw," McClellan said. "If it's not going to get any better then, it won't get any better now. I'll go back to St. Louis and have it looked at."
Rick Eymer is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.