Cavaliers starting guard JR Smith needs surgery on his right thumb and will be sidelined indefinitely.

Smith got hurt late in the first half of Wednesday's game in Milwaukee and didn't return after halftime. The team won't know how long he'll be out until after the operation.

Smith's injury is the latest and most serious medical setback for the defending NBA champions.

Kevin Love, who is having an All-Star season, missed Wednesday's game with a bruised knee and Chris Andersen suffered a season-ending knee injury last week in practice.

After Smith helped them win the title last season, the Cavs signed him to a four-year, $US57 million ($A79 million) contract following a short holdout.