Reynolds was not in Friday's lineup, but said he hurt the muscle making a throw during batting practice.
"It's not too bad, but it's one of those things where if I push it it'll get worse," Reynolds said. "So I'm just going to get it better before anything else happens."
Reynolds said the training staff has done numerous tests on him, and that the oblique feels stiff and sore.
Reynolds was optimistic about his chances of returning to the lineup at the end of the 15-day period, but manager Buck Showalter's outlook did not seem as positive.
"We'd feel real good if he made the DL period," Showalter said. "Those things are usually pretty lengthy."
The Orioles dealt with a similar situation last season, when JJ Hardy missed 27 games between April 7 and May 8 with a strained oblique.
Showalter said the team waited three or four days after Hardy's injury and thought it had healed, only to have it reaggravate with the first hard swing he took.
"His was worse last year," Reynolds said. "I'm not looking at that timeframe right now. I want to push it as much as I can and do as much treatment as I can and try to be back within the 15 days."
This is Reynolds' first trip to the DL in his six-year Major League career.
Reynolds has played third base, first base and designated hitter for the Orioles this season, batting .191 with two home runs and nine RBIs. After a slow start at the plate, Reynolds had started to show signs of a turnaround, hitting .308 with both home runs coming in May.
"It seemed like I was swinging it well, seeing the ball well and then this happens," Reynolds said. "But it is what it is. You roll with the punches and hopefully I'll come back healthy and better."
"I do know a lot of treatment, a lot of rest and we'll get it better."
Reynolds is just another on an increasingly growing list of players the O's have had to send to the DL, leading to a bizarre series of roster moves this week.
Injuries have combined with a rigorous schedule that included a 13-inning game May 4, a 17-inning game Sunday and a doubleheader on Thursday to force juggling of the 40-man roster that Showalter said has been the club's biggest challenge.
When the Orioles officially remove someone from their 40-man roster to make room for Bill Hall, it will be the team's 20th transaction since May 4.
Eight players -- Reynolds, Matt Lindstrom, Endy Chavez, Nolan Reimold, Brian Roberts, Zach Britton, Taylor Teagarden and Tsuyoshi Wada -- are currently on the disabled list.
"We're looking today at like seven guys we were counting on to make our 25-man roster," Showalter said. "It's getting a little crowded in Sarasota."
Greg Luca is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.