"He's got a little something bothering [him] that we think should manage by Tuesday or Wednesday," O's manager Buck Showalter said of Gausman, who started the season on the disabled list with tendinitis.
Dylan Bundy, who would have been pushed back if Gausman was healthy, will pitch on Sunday in the Orioles' regular-season home finale.
Showalter said Gausman didn't feel it in his last start on Tuesday, in which he went 6 1/3 innings and allowed five runs to the Red Sox, which was more than he had given up in his past five outings coming in.
Gausman is 8-11 with a 3.57 ERA in 28 games, all starts, this season. Ideally, the O's would like to line up the rotation so Tuesday's starter is available for Game 162, though Showalter acknowledged it may not be possible to do that.
"Ideally that's the way you'd do it. But where we are with some of the physical challenges we have, we may not have the luxury," he said. "Gaus felt a little bit of an intercostal. We think we caught it in time, but we'll see how it manages. We just don't think it's smart to have him pitch on Sunday."
Reliever Jayson Aquino is also unavailable with a viral infection and the Orioles may have to add another lefty to the bullpen, Showalter said.
Brittany Ghiroli has covered the Orioles for MLB.com since 2010. Read her blog, Britt's Bird Watch, follow her on Facebook and Twitter @britt_ghiroli, and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.