Millsap was knocked to the floor in a collision with Brooklyn's Earl Clark with 1:52 remaining in the first half. The Hawks say Millsap will not return to the game.
The players were chasing a pass intended for Millsap when the collision knocked Millsap to the floor. No foul was called, despite a plea from Hawks coach Mike Budenholzer.
Millsap was escorted to Atlanta's bench and then to the locker room before the end of the half.
The Hawks provided no immediate details on the injury. Mike Muscala opened the second half at Millsap's forward spot.