"Yesterday was the first time I really felt a pinch," Duensing said. "That's the best way I can explain it. I knew it was more serious than I thought it was originally."
But Duensing, who had never been on the DL in his seven-year career, said the injury is still considered minor, and that he expects to be activated once his 15 days are up. The plan is for him to a rest for a bit before starting a throwing program.
"We should be able to go within the 15-day DL time frame," Duensing said. "I think we'll take a week to take it easy and then I'll start throwing a baseball and see how it feels. But we're feeling pretty positive because we were able to keep it at bay for three days without any pain, and so if we give it time to wind down, we should be all right."
Thielbar was a mainstay in Minnesota's bullpen the last two seasons, but didn't make the club after struggling in Spring Training. He threw 2 1/3 scoreless innings for Rochester before being called up.
Thielbar, 28, has a career 2.59 ERA with 74 strikeouts in 93 2/3 innings with the Twins. He had a 3.40 ERA with 35 strikeouts in 47 2/3 innings last year, but had a 4.15 ERA in the second half of the season.
Left-hander Aaron Thompson still remains ahead of Thielbar on the depth chart and will take Duensing's previous role as a late-inning lefty.
"We're going to have to make some adjustments," Twins manager Paul Molitor said. "I thought with Cleveland's lineup and their personnel, calling up a lefty up here was the right thing to do. So we did that with Thielbar."
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.