The decision means the Cardinals are keeping Mitchell Boggs on the active big league roster for now despite a 12.66 ERA.
"In Boggs' case, he's had some tough luck. But I also think he knows he has to get it done," general manager John Mozeliak said. "Clearly, we're going to try to be patient with it. But it's a legitimate question. For how long, I don't know."
Boggs was one of the top setup men in the National League last season and began this year as the stand-in closer for injured Jason Motte. Boggs was demoted after blowing two of four save chances, with most of his troubles compacted into three outings in which he permitted 13 runs in 1 1-3 innings and took two losses.
Boggs was charged with three runs in two-thirds of an inning in a 9-0 loss Sunday to the Pirates, although the Cardinals saw positive signs.
"I know no one here's going to agree with it, but yesterday was as close as we've seen him for a while," manager Mike Matheny said. "One thing compounded into another but he's close.
"It doesn't help us, doesn't help him, because close doesn't really work. We need to get him right and hopefully he finds it soon."
St. Louis selected the contract of right-hander Seth Maness from Triple-A Memphis, and he joined the team in time for Monday night's game against the Reds. The 24-year-old Maness, 2-2 with a 4.32 ERA in five starts in the minors, was the franchise's minor league pitcher of the year last season and will be making his major league debut.
The moves leave the Cardinals with one lefty in the bullpen, Randy Choate. Maness is a candidate for long relief.
Rzepczynski allowed seven runs in eight innings over nine appearances for a 7.88 ERA. He gave up a two-run homer to Russell Martin in the ninth inning Sunday.
"We were very much in agreement that there's a couple things off," Matheny said. "He's going to hopefully get a consistent chance to pitch and find it."