Twice All-Star Horford, 27, left the court with a right shoulder injury late in Thursday's 127-125 double-overtime win against the Cleveland Cavaliers and did not return.
A magnetic resonance imaging (MRI) exam and medical examination in Atlanta on Friday revealed a complete tear of his right pectoral muscle.
"We will work with Al and complete our due diligence to determine the next best steps for him," Hawks general manager Danny Ferry said in a statement. "Injuries are part of the challenge of an NBA season."
The Hawks, who are 16-13 this season and sit third in the 15-team Eastern Conference, said that Horford was ruled out indefinitely and that an update on his status would be given "at the appropriate time".
Dominican Republic native Horford, an All-Star in 2010 and 2011, is averaging 8.4 rebounds and 18.6 points on 56.7 percent shooting from the floor this season.
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