Wellemeyer (7-9) has just one win since June 17 and has given up at least four earned runs in each of his past three starts. The Cardinals made the move in part because nine off-days in the final two months of the season means they will need a fifth starter just six or seven more times this season.
Rookie Mitchell Boggs will be recalled from Triple-A Memphis to take Wellemeyer's spot in the rotation on Friday against the Houston Astros, but a timetable wasn't given beyond that.
"[Wellemeyer's] spot doesn't come open after those off-days for a while, so we will just play it by ear as we go along," said manager Tony La Russa. "We don't lack any confidence in what he's capable of doing, but he's been struggling and there is different things you can do when a guy is struggling and this is one of them. I think he understands."
The Cardinals have lost three consecutive games started by Wellemeyer, who has given up five or more earned runs in seven of his 20 starts and has had an ERA above 5.00 since May 7.
"I'm going to make it a good thing for me, that's all there is to it," Wellemeyer said. "There's no other option. There's no other way to look at it.
"It will be different, that's for sure. I haven't done it in 400-something innings. We're just going to see what happens. We have to just switch up and see how it goes."
Wellemeyer made one appearance in relief for the Cardinals before the All-Star break on July 12 but hasn't pitched exclusively out of the bullpen since 2007. He has made 125 career relief appearances for the Cubs, Marlins, Royals and Cardinals since entering the Major Leagues in 2003.
He holds a career record of 6-9 and a 5.32 ERA out of the bullpen, almost a run higher than his career ERA as a starter of 4.36.
"The way the schedule works and keeping everybody else on a regular schedule makes sense," Wellemeyer said. "It will be fine. I'll get some good appearances out there and hopefully throw up some zeros and feel good.
"I feel good. But instead of just feeling good, I have to pitch good. I have the feeling good down, so now we're just going to go after pitching and try to dominate. The first half of the year I felt terrible and couldn't get anything and now I feel great, as far as mechanics go. I just have to make the best out of it."
Boggs, who was scratched from his scheduled start for Memphis on Tuesday night, gave up just one earned run in his last three starts before learning of his promotion.
B.J. Rains is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.