The fracture occurred during Wednesday night's preseason game. McGary played 36 minutes and scored 14 points in the loss to the Denver Nuggets.
The Thunder chose McGary with the No. 21 pick in May's 2014 NBA draft.
McGary had back surgery in January and played just eight games as a sophomore at Michigan last season. He declared for the draft in April after finding out he was facing a one-year suspension for testing positive for marijuana during the NCAA tournament, which he did not participate in due to his injury.
In his freshman season at Michigan, McGary averaged 7.5 points, 6.3 rebounds and 1.1 steals per game.
The 6-foot-10 forward averaged 14.8 points, 5.8 rebounds and 1.8 blocks in four summer league games for the Thunder.
Oklahoma City opens the regular season on Oct. 29 against the Portland Trail Blazers.
- Molly Geary