Millsap was knocked to the floor in a collision with Brooklyn's Earl Clark with 1:52 remaining in the first half.
"We'll evaluate it and know more (Sunday) and Monday,'' Hawks coach Mike Budenholzer said.
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 Budenholzer.
"I thought he got ran over, but it was a loose ball and both guys were going after it,'' Budenholzer said.
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.
Information from The Associated Press was used in this report.