After the game, manager Jim Riggleman said it was too early to know if Olsen was going on the disabled list.
Olsen, who was not available for comment, pitched three innings, allowing four runs on six hits, one of them a two-run homer to Adam Jones in the second inning. An inning later, he allowed a two-run single to Craig Tatum.
Olsen was scheduled to hit in the bottom of the third inning, but Livan Hernandez came in as a pinch-hitter and struck out.
"He was not feeling good from the bullpen [session] on," Riggleman said. "It was just tightness in his shoulder. He was trying to battle through. He gave us everything he had. It was a very gutty performance. I just decided to get him out of there."
Pitching coach Steve McCatty and catcher Ivan Rodriguez knew something was wrong with Olsen during his bullpen session, but Olsen insisted he was OK. But McCatty warned Riggleman that something wasn't right.
"I wasn't sure if he was completely loose," McCatty said. "He is a tough kid. He wouldn't say anything. He went out there and battled. The shoulder bothered him. The kid said he was all right, left it at that. I appreciate the fact that he tried to toughen it out if the injury was really bad. The guy was pretty tough. It showed me a lot."
Olsen was hit hard during his final two innings on the mound.
"In the bullpen, I asked him if he was OK. I was screaming at him, and he didn't say anything," Rodriguez said. "But I knew something was wrong with him. Hopefully it's just for a few days and he could come back in his next start and be healthy. It was tough for him today."
The injury comes at the worst time for Olsen, one of Washington's best pitchers, who entered the game with a respectable 3.15 ERA.
Olsen is scheduled to pitch on Thursday against the Giants, but the team will most likely have to make a decision before then.
This is not the first time that Olsen has had problems with his left shoulder. He missed the entire second half of the 2009 season after having shoulder surgery.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.