Thompson left Monday night’s loss to Milwaukee in the third quarter after landing awkwardly on Bucks guard Malcolm Brogdon’s foot.
The team said in a statement that an MRI and further evaluation Tuesday confirmed Thompson’s sprain and he would begin treatment and rehab.
The Cavaliers are still without All-Star forward Kevin Love, who could be sidelined until January following toe surgery.
Thompson, 27, is averaging 12 points and 11.6 rebounds per game for the Cavaliers. Cleveland dropped to 6-21 with the loss to Milwaukee.