"When I first heard about it a couple days ago that he was having some tenderness in there, he hadn't pitched for a couple days, so we obviously know that it's something that you can't take lightly," Twins manager Paul Molitor said. "He said he maybe wanted to try to pitch through it and we said we got to get it checked out, and we did that today."
Molitor said Boyer had no structural damage. He will stay back while the team embarks on its upcoming road trip, though if everything goes well, he might be able to rejoin the Twins in Tampa Bay.
Boyer excelled at the beginning of the year, taking over as the team's setup man. His performance tailed off and he has gradually been working in earlier innings, but Molitor doesn't think the injury was a factor in the change in performance.
"I think it's something that's kind of come on recently and he thought he needed to address," Molitor said.
In the meantime, the Twins recalled Tonkin for the fourth time this season. Tonkin has pitched in 16 games for the Twins this year, posting a 5.73 ERA in 11 innings.
Tonkin has given up three earned runs in 23 1/3 innings since he was last sent down in late June. In that time, opponents are hitting just .171 against him.
"I honestly didn't change a whole lot, I don't feel like," Tonkin said. "I just went down there and just had the mentality to get guys out."
"The two areas that jump out for me are improved consistency of his breaking ball, and he's got to be better at holding runners and containing the running game," Molitor said. "His fastball plays, it's just that when that's the only pitch that plays, you can still get in trouble. He's been working on those things and hopefully it translates."
Betsy Helfand is an associate reporter for MLB.com.