Horford suffered the injury in the Hawks' 96-84 loss to Indiana on Wednesday.
Atlanta has been one of the league's early surprises. The Hawks beat both the Heat and Bulls last week, arguably the two best teams in the Eastern Conference.
Horford has been one of the key reasons for the Hawks success, both this season and in their four straight playoff appearances from 2007-11. He has averaged at least 10 points and nine rebounds each season of his four-year career, and was off to a solid start to the 2011-12 season, averaging 12.4 points (on 55.3 percent shooting) and 7.0 rebounds in 31.6 minutes per game.
Zaza Pachulia picked up Horford's minutes after he was removed early from Wednesday's game. Pachulia scored 11 points and pulled down seven rebounds in 30 minutes.
Initially, Horford's injury was called a strained shoulder.
TNT's David Aldridge confirmed the report on Twitter.