An MRI on Monday confirmed the sprain. Roberson's injury will not require surgery. After three weeks, he'll be re-evaluated to determine his return to play.
Roberson, 24, injured the knee Sunday in the second quarter against the Brooklyn Nets when Russell Westbrook crashed into his leg following a layup attempt. Roberson exited the game and did not return.
Kyle Singler started the second half in place of Roberson on Sunday.
Roberson has started 45 games for the Thunder this season at shooting guard, averaging 4.9 points and 3.3 rebounds in 21.7 minutes per game. Regarded as a defensive stopper, the Thunder are allowing just 97.5 points per 100 possessions this season with Roberson on the floor.