Celtics forward Gordon Hayward has entered the NBA’s concussion protocol after colliding with Atlanta forward John Collins in Saturday's game at TD Garden.

Hayward is listed as doubtful for Monday’s game against the Nuggets.

The injury occurred as Collins was setting a screen late in the first quarter. Collins’s shoulder struck Hayward in the head area. Hayward appeared to be dazed and was quickly replaced by Jaylen Brown. He was diagnosed with a strained neck and sent home at halftime.

Meanwhile, Celtics forward Al Horford (knee soreness) is listed as probable for Monday’s game. Horford bumped knees with a Hawks player and missed a large chunk of the second half but was eventually able to return to the game.