Carpenter, on the 15-day disabled list since April 15, suffered the injury in his second start of the season.
After missing virtually all of 2007 and 2008 following Tommy John elbow reconstruction surgery and shoulder trouble, the former National League Cy Young Award winner had allowed just one unearned run in 10 innings before leaving his April 14 start after three innings.
At the time, the Cardinals medical staff predicted a recovery timetable of about 30 days, but as recently as a week ago, Carpenter was still experiencing pain in his rib cage.
That has improved significantly.
"The pain is gone," said Carpenter on Thursday, who was with the club in Washington. "I still feel it in real extreme positions at times, but all my everyday stuff is back to normal."
Carpenter, who turned 34 this past week, said he's been running at full speed on the treadmill as well as doing outside laps, without pain. He's scheduled to see the team doctor on Monday and hopes to get clearance at that time to start playing catch.
Lisa Winston is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.