After exiting Sunday's game with the injury, Chen had an MRI on Monday's off-day to confirm the injury, and he said he is focused on returning as quickly as possible. Oblique strains can be tricky and recovery typically goes longer than the minimum 15 days, making it tough to predict how long Baltimore will be without Chen.
"It's one of those things that's tough to handicap the severity of it," Orioles manager Buck Showalter said. "There's three grades, and I think his was in the middle.
"It's so hard to test it. Testing it might make it not heal as proper as it should. ... I don't know. I'm thinking best case scenario. As I said before, if you look at the history of these things, it gives you the idea that it is a little later than sooner. We'll take whatever comes our way."
In Chen's place, reliever Alex Burnett was recalled from Triple-A Norfolk for the third time this season, and the team added infielder Yamaico Navarro, replacing reliever Mike Belfiore, who was optioned on Monday.
The team will need a starter for Saturday in Chen's place, and Orioles executive vice president of baseball operations Dan Duquette -- who has stockpiled starting pitching since coming to Baltimore -- said Monday that he felt confident with the organization's internal options.
Triple-A Norfolk's scheduled starter Saturday was Jair Jurrjens, and it's changed to TBA. Jurrjens is expected to be added to the roster and make his debut for Baltimore that day.
"Jurrjens has pitched well at Triple-A. He's throwing his pitches for strikes, and he's pretty solid," Duquette said of the right-hander, who has a 3.14 ERA in eight starts. "He's ironed out his delivery."
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, and follow her on Twitter @britt_ghiroli. This story was not subject to the approval of Major League Baseball or its clubs.