Alfredo Simon said he felt "a little pop" in his hamstring before leaving the game in the 10th inning.
"I just tried to run hard to the base. We'll see how fast I can recover," he said.
"I think it'll be good in a couple of days, maybe just a week."
The Orioles can't go that long without him, so they could be placing two relievers on the disabled list before Tuesday's game.
Though manager Dave Trembley said Simon was injured while reaching for Luke Scott's throw, I thought I noticed him pull up earlier, and that's how he explained it after the game.
"It happened before that. It happened when I started running. It was before the throw," he said.
Simon has injured his hamstring previously and said he recovered in about two weeks.
"I don't know what's going to happen with this one. I think I'll come back soon," he said.
"We'll see what happens with the doctor, the treatment."