BALTIMORE -- The Orioles lost their best pitcher this season for at least 3-4 weeks, with right-hander Jason Hammel announcing on Sunday that he has decided to undergo right knee surgery to alleviate the growing discomfort that forced him to exit Friday's start early.

"Obviously, it's not what I want to do," Hammel said. "[It's] what needs to be done. I can't keep pitching on a bad leg, start feeling things with my arm and hurting stuff there. Obviously, I'm bummed out."

The Orioles placed Hammel on the 15-day disabled list on Sunday, recalling right-hander Steve Johnson, who was 3-6 with a 3.11 ERA for Triple-A Norfolk.

The team is still working on solidifying an extra surgery slot, but the procedure is expected to be performed by team orthopedist Dr. John Wilckens sometime on Monday morning at Johns Hopkins Hospital. Hammel said the initial timetable for his return is 3-4 weeks, although manager Buck Showalter is taking the approach that he won't have Hammel back until early September.

"I think it is smart for me, personally, to plan that way. Then, if you get your birthday present early ... I'd rather look at the worst case," Showalter said. "I think I'll have a better feel for it when Dr. Wilckens gets out of there and [reports] what he found."

While he has been dealing with soreness in his right knee for more than two months, the team was optimistic Hammel would be able to avoid surgery until the offseason. But he came up limping in the fourth inning of Friday's game, and Saturday's MRI showed that the cartilage in Hammel's knee had shifted to an area that caused more discomfort on the joint.

"It's not easy [to go with surgery], because I could rest for maybe a week and it might go back to where it was," Hammel said. "[But] then you run the risk of it happening every time I go out there.

"It's a decision that had to be made. Obviously, I want to help the club, but it's another opportunity for someone to help out. I'll get back to doing my thing when I get back."

The loss of Hammel is a big one for an already uncertain Orioles rotation, with rookie starter Wei-Yin Chen the lone member of the Opening Day rotation to remain on the team's active roster. A Final Vote candidate for the All-Star Game, Hammel has been the team's best starter at 8-6 with a 3.54 ERA, taking the loss on Friday after yielding two runs on five hits before leaving with the injury.

While his replacement in the rotation was originally thought to be Brian Matusz, Showalter said that Matusz would make Sunday's scheduled start for Norfolk, which removes him from the running. Tommy Hunter is the next logical choice, with his next start for Norfolk already scheduled for Wednesday.