Carpenter, 37, underwent surgery in July to relieve nerve compression that resulted in numbness on much of the right side of his body. The procedure was thought to be season-ending.
But the right-hander made four simulated starts recently without any incident, and is ready to join the rotation as the Cards make a playoff push.
Last season, Carpenter went 11-9 with a 3.45 ERA and played a key role in helping St. Louis win the World Series.
The Cardinals hold the second NL wild-card spot after splitting a four-game series in Los Angeles with the Dodgers.
Information from The Associated Press was used in this report.