Carpenter was injured grounding out to third base to end the top of the fourth. He went to the mound to warm up before the bottom of the inning, then called for a trainer.
"I felt it just a little bit on that swing, and every warmup pitch got worse and worse until the last one bit pretty good," Carpenter said after the Cardinals' 7-6 loss to the Arizona Diamondbacks in 10 innings. "So there's nothing I could do about it but come out."
The Cardinals said they would wait until Wednesday before deciding whether to place the oft-injured Carpenter on the disabled list. He's made eight trips to the DL with shoulder and elbow injuries since 1999.
"Obviously, it's a frustrating spot to be in right now, but our hope is it's not going to be that serious," St. Louis general manager John Mozeliak said. "And most importantly, it's not his elbow or shoulder."
A glum Carpenter would not speculate on how long he might be out.
"I have no idea," he said. "I've never done it before. I know it's pretty sore now."
The Cardinals led 3-0 when Carpenter was removed. The 2005 NL Cy Young Award winner allowed four hits in three innings, striking out two. He has not allowed a run in 10 innings this season.
Asked how it felt to go to the mound to lift Carpenter, manager Tony La Russa replied, "You can imagine. What do you want me to say? Dramatize it? I think everybody understands. I've talked enough about how important he is."
The 33-year-old right-hander made only four appearances last season after reconstructive elbow surgery. He underwent another procedure in November to transpose an elbow nerve that was irritating the muscles in the back of his shoulder and around his neck.