Blackburn, who won 11 games during his rookie season, woke up Feb. 28 to find his left knee swollen, one day after a two-inning spring debut against the Yankees.

The right-hander underwent a program of compression sleeves and constant ice, in hopes of calming whatever irritation remained in the joint, and skipped his next turn to pitch.

The Twins also announced that Blackburn, who had arthroscopic surgery in October, would be held out of fielding drills and running for the rest of the spring.

"It's disappointing, because I thought I had taken precautions against this," said Blackburn, who led all rookies in innings pitched last year. "I'm tired of messing with this already."

The problem isn't pain, Blackburn said, but fluid buildup.

"It feels fine, normal. But whenever I get that fluid in there, my quad won't fire the way it's supposed to," he said. "That's when it throws everything out of whack."

Blackburn threw a bullpen session on March 5 and reported no lingering effects. Still, with no logical replacement ready to move into the rotation behind their five returning starters, the Twins are being careful.

Blackburn hopes to return to action by the middle of the month.