The Celtics said Horford, who missed nine games with a concussion last year, began experiencing headaches Wednesday morning, the first symptoms linking him to a possible concussion.
Horford was struck in the head on a foul called in the second quarter of the Celtics' win at the Atlanta Hawks on Monday night. On the play, Atlanta's Kent Bazemore came down hard with a forearm to Horford's head.
Horford grabbed at his head and stayed on the ground for a bit, but he was able to play on.
However, Boston's medical staff had stressed in the aftermath of Monday's game for Horford to contact them if he had any issues resulting from the hit.
While Horford was spotted in the Celtics' locker room before Wednesday's contest, coach Brad Stevens stressed that they would tread carefully with him.
"We will obviously do whatever is in the best interest of Al," Stevens said. "That's the most important thing."
"I don't expect [Morris] to be Al. I don't expect anybody to be Al," Stevens said. "I think we have to replace what he does by committee. We can't expect anybody to play like him or be like him on both ends of the floor. We just have to make sure that, if he's averaging nine rebounds, we've gotta get nine rebounds out of that spot. If he's scoring the ball for us at a clip, we either have to score it from that spot or around him a little bit more.
"We've been good about focusing on the task at hand and guys being prepared and ready. But there's no question he's a big part of our team on both ends."