Horford first started exhibiting symptoms Wednesday morning after he was struck in the head on a foul called in the second quarter of the Celtics' 110-107 win over the Atlanta Hawks on Monday night. He stayed in the game and had 15 points, 10 rebounds and nine assists against his former team.
The Celtics said the 31-year-old Horford will be monitored as he progresses through the protocol.
The 6-foot-10, 245-pound Horford missed nine games last season due to a concussion. He is averaging 14.6 points, 9.2 rebounds and 4.7 assists in 11 games this season.