Lawrie, who is batting .247 with 12 homers and 38 RBIs had just come back after missing more than a month with a broken right index finger when he left Toronto's loss Tuesday to Baltimore.
Lawrie underwent tests the following day, which confirmed the injury. He was placed on the 15-day disabled list Thursday, retroactive to the previous day.
"Certainly Brett's going to be out a while," Blue Jays general manager Alex Anthopoulos said. "I think it's safe to say September would be the earliest he's going to be back."
The Blue Jays recalled left-handed reliever Rob Rasmussen from Triple-A Buffalo. Infielder Steve Tolleson is expected to return from three days of paternity leave Friday, when Toronto begins a three-game series against AL Central-leading Detroit.
The news was better for the injured duo of Edwin Encarnacion (strained right quadriceps) Adam Lind (right foot). Anthopoulos said Lind, who doubled twice in two at-bats Thursday in his first rehab game, is "days away" from a return and could join the team in time for Monday's game at Seattle.
"He feels he's really close," Anthopoulos said.
Encarnacion felt good after testing his sore leg by running hard Thursday, and could begin a rehab stint this weekend, Anthopoulos said, adding that he hopes to have the slugger back within the next week.
This is the third oblique injury of Lawrie's four-year career. He strained his left oblique in a warmup game for the 2013 World Baseball Classic last March, missing the first 14 games of the season. Anthopoulos said MRI results indicated the current strain was a new injury, not a recurrence.
Lawrie also missed almost a month with a right oblique strain in late 2012. This is his sixth career trip to the disabled list.
Besides Tolleson, the Blue Jays have Juan Francisco, Munenori Kawasaki and Danny Valencia to handle third base. With Toronto currently holding down the second AL wild-card berth, Anthopoulos said he'll continue to explore moves to shore up the infield.
"We're down a right-handed bat, and if we can find someone along the way in the next few weeks, we'll look to do that as well," Anthopoulos said.
Also Thursday, the Blue Jays signed catcher George Kottaras to a minor league contract. He will report to Buffalo.