Dempster suffered the injury during a bullpen session earlier this week. After undergoing an MRI on Friday, the decision was made to skip him for at least two turns through the rotation.
"He's much, much better, but he still feels it," manager Dale Sveum said. "[The move is] just precautionary. Let him miss a couple of starts and hopefully end up with 31-32 starts when the season's over -- if this thing doesn't linger and he's able to pitch the way he's capable of pitching."
The move was retroactive to Wednesday, making Dempster eligible to return during the first week of May. He has been the Cubs' top starting pitcher during the early part of the season, posting a 1.33 ERA over three starts. He's also tied with the Phillies' Cole Hamels and the Dodgers' Aaron Harang for the National League lead in strikeouts with 23.
Veteran righty Randy Wells will fill Dempster's spot in the rotation. His first outing will come in Sunday's series finale against Cincinnati. In three starts for Triple-A Iowa, Wells was 1-0 with a 9.42 ERA.
Wells was not added to the roster immediately, though he rejoined the club on Friday and was in the clubhouse again on Saturday. Sveum has high hopes for Wells' outing on Sunday.
"Nine-inning shutout perfect game would be nice," Sveum joked. "He's a veteran guy that knows how to pitch. Sinks the ball and has a good changeup."
Taking Dempster's place on the roster for Saturday's game was speedy outfielder Tony Campana, who was recalled from Iowa. Campana hit .259 in 95 games for the Cubs last season and led the club with 24 stolen bases in 26 attempts.
At Iowa, Campana was hitting .304 with eight stolen bases in 14 games. Sveum said he'll use Campana off the bench for the time being, though starting center fielder Marlon Byrd has struggled badly this season, hitting .070 over his first 43 at-bats.
"We'll just see," Sveum said. "We'll play him off the bench and pinch-run. Whatever way we can get his speed in the game, we'll do our best to do that."
The Cubs said another roster move will be made on Sunday to accommodate Wells.
Bradford Doolittle is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.