Horford left during the first overtime of Thursday night's 127-125 double-overtime win at Cleveland and did not return. The Hawks said an MRI and examination on Friday by Dr. Michael Bernot in Atlanta revealed the complete tear of the 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."
For the Hawks (16-13), the loss of Horford for the season or even for an extended period could be devastating. Following Josh Smith's exit to Detroit as a free agent, Horford has taken leadership with point guard Jeff Teague and power forward Paul Millsap.
Horford has proved to be a good fit in first-year coach Mike Budenholzer's offense. He leads the team with 18.6 points and 8.4 rebounds per game.
It's uncertain how the team will adjust to Horford's injury. Pero Antic, who replaced Horford Thursday night, may be the most likely candidate to move into the starting lineup.
Antic has averaged 3.6 points while playing 10.5 minutes per game.
Horford, in his seventh season from Florida, set career highs with his averages of 17.4 points and 10.2 rebounds last season.
Forward DeMarre Carroll is day to day with a sprained right thumb.