Horford, the Hawks' leading scorer at 18.6 points per game, was diagnosed with a complete tear of his right pectoral muscle after missing the club's last three games.
He was injured during Atlanta's 127-125 double-overtime win at Cleveland Thursday.
Horford leads the Hawks in rebounding at 8.4 per contest, as well as in field goal percentage (.567) and blocks (1.5).
Elton Brand has taken over at starting center for Atlanta, which holds third place in the Eastern Conference at 18-14.
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