The injury is believed to have occurred in Monday night's game against Atlanta. The Celtics say he showed no symptoms until Wednesday morning, when he entered the NBA's concussion protocol.
Horford has averaged 14.6 points, 9.2 rebounds and 4.7 assists this season, taking on an even bigger role since the injury to Gordon Hayward on opening night. The Celtics have won nine games in a row.
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