Coach Tony Dungy said Wednesday that Brackett would be replaced in the lineup by little-used Buster Davis. Indianapolis signed Davis in September after he was waived by the Detroit Lions.
Brackett injured his leg in the second quarter Sunday against the Cleveland Browns when he wound up at the bottom of a pile.
The loss of Brackett could have a significant impact on the Colts' improving defense. Brackett is the defensive captain, makes all the play calls and wears the new radio helmet. Dungy said it was like losing a quarterback.
Brackett's absence is yet another blow in a rash of injuries for Indianapolis.
Jeff Saturday, the three-time Pro Bowl center, is expected to miss his second straight game with a strained left calf, and the Colts have already lost starting cornerback Marlin Jackson, backup running back Mike Hart and left guard Ryan Lilja for the season with knee injuries.
Davis has played in only two games this season and has been inactive for eight of the past nine weeks.
"It's a big loss," said DE Robert Mathis. "He's our captain, he gets everybody in position. But the next guy just has to step up."