Horford escaped injury on the play but it's exactly the sort of foul that can really harm a player.
As he was in midair, on his way to an alley-oop slam, J.R. Smith shoved Horford in the back, sending him crashing to the ground.
Smith was assessed a flagrant foul and both received technical fouls.
With his Cleveland Cavaliers visibly frustrated late in Tuesday night's Eastern Conference Finals series against the Boston Celtics, the always enigmatic J.R. Smith might have committed the dirtiest foul of the playoffs. That's not unusual for a younger group such as the Celtics, and the Cavs are right to expect a shift in bravado and tenacity with Games 3 and 4 at Quicken Loans Arena beginning Saturday.
On the play against Horford, Smith couldn't deny what he did.
Smith pushed an airborne Horford in the lane and drew a flagrant. He can't control how his body goes, and he's not even looking, and you go and take two hands to the back.
"That's not the first time J.R. has done some dirty stuff, especially playing against us", Smart said. "It wasn't like I was trying to lowbridge him or something to make sure he didn't get it".
Smith had a awful game, recording zero points while going 0-for-7. "If you let a bully keep picking on you, he's going to keep picking on you until you finally stand up".
The flagrant gave Horford two shots and the Celtics the ball back. James, who took a blow to the jaw from Jayson Tatum's left shoulder in the second quarter and briefly left the game, and Love combined for 64 points.