Slowey saw a local specialist, Dr. Brad Pierce, who ruled out a sports hernia but recommended seven to 10 days of rest.
Manager Ron Gardenhire said after the 3-0 loss to the Seattle Mariners that Slowey will be placed on the disabled list and another relief pitcher will be brought up from Triple-A Rochester. Gardenhire said the pitcher would be named on Friday, but it likely will be right-hander Jim Hoey.
Slowey has made only six appearances this season, the last on Friday at Arizona, and posted a 4.91 ERA in 14 2/3 innings. He's been bothered by injury problems and has partially blamed them on the adjustment from starting to relieving.
The latest issue popped up after he pitched on May 15 against Toronto. Team physician Dr. John Steubs recommended rest and ordered an MRI on Monday, then sent him to the specialist.
Slowey said he feels some discomfort walking around. With an injury to the core muscles like this, he'll have to take it easy.
"I don't plan on doing a whole lot of heavy lifting certainly," he said. "The emphasis really was on rest."
With Slowey out and closer Matt Capps also unavailable for another game because of soreness in his forearm caused by overuse, the Twins took the field against the Mariners with an understaffed bullpen.
Gardenhire said he didn't believe an MRI was necessary for Capps, but he said the team wanted to "back off" from using him until later in the week.
"It's the right thing to do," Gardenhire said.
Capps has entered the game in the eighth inning five times this month, and Gardenhire said he wants to get back to using him exclusively in the ninth inning.
"We really like the ideal of bringing him in the ninth inning and letting him have a clean inning," Gardenhire said. "Unfortunately we've had to do a few things. One came with an injury and then the other night we were forced into that one, too. But no we don't want to use him like that. We want him to pitch the ninth and get us there."
When Slowey is ready to pitch again, he will probably be sent to Rochester because he has told the Twins it's difficult for him to adjust to the routine of relieving at a moment's notice rather than taking the mound every fifth day.
But Slowey, who is making $2.7 million this season, won't be forced into Minnesota's rotation, Gardenhire said.
Left-hander Brian Duensing, who started Wednesday's game against Seattle, is most capable of effective relief.
"I'm not going to punish Duensing because one guy can't pitch out of the bullpen, I'll tell you that right now," the manager said, adding: "To me that's wrong. I like Duensing in the rotation. I like the kid. So he's in there."