Carpenter has tear in oblique muscle
Carpenter received an injection and his condition will be re-evaluated next week.
"It just confirmed our initial read," general manager John Mozeliak said, according to the St. Louis Post-Dispatch. "You certainly would have preferred it find nothing. But given his level of pain [Tuesday], this is what was expected."
Carpenter (1-0) had pitched 10 scoreless innings this season until injuring himself grounding out to third base to end the top of the fourth inning Tuesday night against the Arizona Diamondbacks.
He missed virtually all of the last two seasons with elbow and shoulder injuries. He was the NL Cy Young Award winner in 2005.
Mozeliak accompanied Carpenter back to St. Louis on Thursday.
"He said he felt better today, which is very encouraging," Mozeliak told the Post-Dispatch.
To fill the hole in their rotation, the Cardinals on Friday called up right-hander P.J. Walters, who will start Friday's game against the Chicago Cubs.
The 24-year-old Walters impressed Cardinals manager Tony La Russa during spring training and pitched seven shutout innings in his one minor league start for Triple-A Memphis this season.
Walters began the 2008 season in Double-A before getting promoted to Memphis, where he went 9-4 with a 4.87 ERA.
The Cardinals optioned right-hander Josh Kinney to Memphis to make room on the 25-man roster for Walters.
Information from The Associated Press was used in this report.