Bonser strained his lower back on the pitch, spent a few seconds stretching it out on the mound and came back to strike out Melvin Mora on four pitches to escape the inning. But he spent the rest of the night in pain from what was termed a lower back strain.
"They said it was a strain," said manager Ron Gardenhire. "The doctor looked at him and said it wasn't structural, as far as the back goes, it's muscular. ... I can't tell you how long a strain is for a pitcher. He's just going to be day-to-day.
"I saw him stretching out on the mound, but most pitchers do that. ... I couldn't tell, and then he came off the field and said he strained something. He was still throwing the ball good, velocity didn't go down or anything like that -- he didn't not throw a breaking ball or anything like that, so we couldn't tell."
Bonser said he felt remarkably better on Sunday despite suffering through chronic pain all night on Saturday. The right-hander, who has a 6.09 ERA through 109 1/3 innings this season, said he's never experienced any problems with his back before, but doesn't expect the injury to hamper him for very long.
"It wasn't bad, it just locked up on me when I threw," Bonser said. "I've got a little bit of pain right now, very little, so I'm getting ready to go out, run around and play catch and see where we're at. ... I've been getting my treatment done and stuff, so I'll just run around, sweat a little bit and see how it is."
Earlier in the weekend, Bonser was listed as a possible starter for the Twins' game against Cleveland on Wednesday. Gardenhire announced on Sunday that Scott Baker would be the likely hurler to take the mound for that game. Baker will be going on three-days' rest, but Gardenhire cited the right-hander's strength and experience as reasons why the short rest would not be an issue.
Amanda Comak is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.