Horford was injured in Friday's loss at Orlando and is out indefinitely after an MRI showed the bruise, coach Mike Woodson said.
The Hawks said before Sunday's game that Horford will not be with the team for a three-game road trip that begins Tuesday at Phoenix.
He is averaging 10.8 points and leads the team with 8.7 rebounds per game in his second season. Horford, the No. 3 pick in the 2007 NBA draft, was the only unanimous choice on last year's NBA All-Rookie team picked by the league's head coaches.
Zaza Pachulia returned after missing one game with the flu to start at center against Philadelphia.