The team announced he had a right quadriceps contusion. He will undergo an MRI on Thursday, a source told ESPN's Adrian Wojnarowski.
The 6-foot-10 Gafford collided with Celtics forward Jaylen Brown and went to the floor, grabbing his right leg just out of bounds across from Boston's bench midway through the second quarter.
Wizards players in the game and on the bench came and surrounded Gafford as a member of the team's training staff checked on him.
He was helped up and brought straight to the locker room, unable to put any weight on his leg. He did not return.
Brown briefly grabbed his right knee but got up slowly and stayed in the game.
The Associated Press contributed to this report.