"Timmy has been having a little bit of an issue with a right intercostal," Twins manager Paul Molitor said. "He's been treating it for the last week or so. It's one of those situations where it's a guy who pitched with an injury. We've certainly been aware of it with our training staff, but it kind of came to a point where I talked to Timmy and I don't think the results are there. And he knows that, so the best chance for him to help our team is to make sure what he's dealing with isn't inhibiting him from pitching his best."
Molitor said it's too early to know for sure, but said the injury could be the cause for Stauffer's velocity issues early this season.
"I think your mind has a way of protecting your body and if you can't extend and finish your pitches, even if it's just subconscious, it could affect not only your velocity but your offspeed stuff, too," Molitor said. "So we'll find out long term if that's been a factor."
Stauffer, who joined the Twins on a $2.2 million deal in the offseason, was serving as the club's long reliever.
Pressly, 26, made seven relief appearances for Rochester, posting a 4.50 ERA with 15 strikeouts and six walks in 10 innings. He has a career 3.60 ERA, with 63 strikeouts and 35 walks in 105 innings with the Twins over the last two seasons.
"His outings have been a little bit hot and cold," Molitor said. "But [Twins general manager] Terry [Ryan] was impressed with the pitches he was using out there. If he's not throwing it over, that's when he gets in trouble. But the velocity is good and the sharpness is there, it's just about throwing it in the zone. We know he has big league stuff."
Following Friday's 1-0 victory over the White Sox, the Twins optioned left-handers Tommy Milone and Caleb Thielbar to Rochester in order to create roster room for Saturday's activation of right-hander Ricky Nolasco and left-hander Brian Duensing.
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.