FORT MYERS, Fla. -- The Twins will likely be without pitcher Boof Bonser for the entire 2009 season after doctors discovered partial tears in the labrum and rotator cuff in his right shoulder during arthroscopic surgery on Wednesday.

Two previous MRIs performed on Bonser's shoulder had revealed no structural damage. But the tears were discovered and repaired during Wednesday's arthroscopic scope performed by Rays orthopedist Dr. Koko Eaton.

Bonser is expected to miss six to eight months recovering from the surgery. Manager Ron Gardenhire said that he doesn't anticipate seeing Bonser back on the mound this season.

"We're probably looking at out for the season type thing," Gardenhire said. "That's just a best guess guesstimate. ... You are talking some major things that had to be done. So I would say that's probably a season."

Bonser first began experiencing shoulder discomfort at the end of last season. An MRI performed at that time did not show anything, and Bonser was ordered to rest the shoulder and begin a rehabilitation program.

But the treatment didn't help. Bonser tried to throw in early January and felt the same discomfort. The right-hander received a cortisone shot from Eaton just a few days before he reported for Spring Training. The shot had little effect on his shoulder, so the decision was finally made to have Bonser undergo a scope to determine what was causing his shoulder problems.

Prior to the injury, Bonser had appeared to be a lock for one of the Twins' bullpen jobs. Bonser had been slotted in as the club's long reliever, but there was also discussion of perhaps moving him to a late-inning role.

"He's got a good arm, a power arm with a big breaking ball and the whole package," Gardenhire said. "As we said, we could have used him in a lot of different ways. He was in our plans. But we just have to make adjustments.

"When people go down, other people get a chance to step up and it makes an opening for a ballclub that he was probably going to be on."

With Bonser out of the mix, right-handers Philip Humber, who is out of options, and Jason Jones, a Rule 5 Draft pick, are now the favorites to win the final spot if the Twins keep seven relievers. R.A. Dickey, a knuckleballer, and left-hander Sean Henn are also among those expected to compete for the job.

There is a chance that the Twins could break camp with an 11-man pitching staff, meaning just six relievers will make the Opening Day roster. It's a possibility that the coaching staff has discussed this spring, but so far Gardenhire, has indicated that it's more likely the team will open the regular season with 12 pitchers.