With Monday's off-day, the Twins went with a four-man rotation but needed a fifth starter against the Reds with right-handers Carl Pavano and P.J. Walters on the disabled list.
The Twins decided to go with Duensing, who has a 3.12 ERA with 22 strikeouts and nine walks in 34 2/3 innings as a reliever. He last pitched on Tuesday against the Pirates, when he tossed 1 2/3 scoreless innings in relief.
Duensing, who threw a bullpen before Thursday's game, is expected to be restricted to about 50-60 pitches because he hasn't had the chance to build up his pitch count yet.
"We decided to let him stretch out a little bit and piggyback him with a couple other guys and see what happens, how many innings he goes," Gardenhire said. "They got some pretty good left-handed hitters in that lineup, so you try a lefty over there too."
Part of the reason the Twins went with Duensing is because two of the top Reds hitters -- Joey Votto and Jay Bruce -- are left-handed. But Duensing also has improved against right-handers this season, as they posted a .947 OPS against him last year but have a .645 OPS against him this year.
Duensing, though, said he's not sure why he's been so much better against right-handed hitters this season.
"I don't know," Duensing said. "Maybe I'm making some better pitches. When I've gone in, I've gone down and in instead of up and in. I've been throwing strikes in there, and they have to be aware of that instead of disregarding that and look middle away. ... And I've had some good luck on top of it, too."
Duensing, who has made 50 career starts, including 28 last year, said he's hoping his return to the rotation is similar to both '09 and '10, when he joined the rotation to much success after starting out in the bullpen. He posted a 2.73 ERA in nine starts in '09, and had a 3.05 ERA in 13 starts in '10.
"If I could throw like that it'd be great," Duensing said. "I'm really hoping that will happen. I don't know. But hopefully I'll go out there and do well and give the team a chance to win and not hurt the bullpen too bad."