"I'll be in the bullpen for who knows how long and try to make the best of it," Wellemeyer said. "We had to switch it up and see how it goes.
"I'm going to make it a good thing for me, that's all there is to it."
Wellemeyer threw a bullpen session Tuesday before the Cardinals played the Dodgers. Manager Tony La Russa said the pitcher will be available for relief duty beginning Thursday.
"We don't lack any confidence in what he's capable of doing, but he's been struggling," La Russa said. "I think he understands."
The Cardinals plan to recall righty Mitchell Boggs from Triple-A Memphis to take Wellemeyer's turn against Houston on Friday night. Boggs has started four times for St. Louis this season and is 1-0 with a 3.22 ERA.
St. Louis began the day in second place in the NL Central, a half-game behind Chicago.
Wellemeyer went 13-9 with a 3.71 ERA last year in his first full season as a starter. He said he's been impatient to finish off hitters, and end up leaving balls up in the strike zone.
"Mechanically I feel good, and now it's just a matter of just going out there and competing my butt off," Wellemeyer said. "I know what I need to do."
Even if Wellemeyer returns to the rotation, it'll probably be as a spot starter. The Cardinals have nine days off the rest of the season after a heavy early schedule and La Russa plans on going with four starters whenever possible.
Rookie center fielder Colby Rasmus, who had a second cortisone injection for a bruised left heel on Monday, was back in the lineup and batting sixth. Rasmus totaled three at-bats the previous four games, none of them starts.
Rasmus was back in the lineup partly because outfielder Rick Ankiel has a slightly strained groin sustained in a swing on Monday.
Infielder Julio Lugo, 9 for 19 with a homer and three RBIs in four games since joining the Cardinals, was not in the lineup for the first time. Shortstop Brendan Ryan got a second straight start after tying his career high with four hits Monday.