Olsen, who returned to Nationals Park at 9:50 p.m. ET, will have surgery on Thursday morning at the Washington Hospital Center. Dr. Wiemi Douoguih, the team's medical director, will perform the procedure.
"Nine times out of 10, it is just a cleanup procedure with the idea that [the player is] back -- competition-ready -- within three months. That's the goal here," Douoguih said. "Every once in a while, you go in and you find something a little bit more extensive. Probably 90 percent of them are just going to be a cleanup procedure."
Olsen appeared to be in disbelief that he would no longer play in 2009. He hopes to be ready by Spring Training.
"It's surprising," Olsen said. "We had an MRI done the other day. We got Dr. Wiemi's opinion, and today we flew to Alabama and got Dr. Andrews' opinion. Everybody seems to be along the same line of thinking, so we are going to go ahead and have some surgery done. Hopefully, it will work out well and get ready for next year."
Olsen started feeling pain in the shoulder on July 10, against the Astros in Houston. In that game he lasted six innings and gave up four runs in a 6-5 loss.
"I just didn't feel normal," he said. "The day after I pitched in Houston was a hard day. I really couldn't go through my throwing program like I normally would. It didn't get any better with the days off during the All-Star break. It didn't get any better. I had to skip Friday and then get second opinions. I'm not the only person that has had surgery. We just go back and try to build up for next year. "
Olsen also missed time earlier in the year because of a sore shoulder. He is 2-4 with a 6.03 ERA this season.
"I thought this was behind me. Obviously, it's not. There's nothing else I can do about it now." he said.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.