Brown suffered the injury in the second quarter and will undergo surgery Monday.
The injury is the latest for the Seahawks, who last week lost starting quarterback Trent Dilfer for the rest of the season with a torn Achilles tendon and receiver Darrell Jackson to a concussion and seizure.
Seattle came into Sunday's game ranked last in the NFL against the run and 30th overall in defense. But Brown has been one of the few bright spots, leading the club with seven sacks in eight games.
Earlier this season, Brown signed a five-year contract extension worth a reported $28.5 million.
One of the league's better passing rushing linebackers, Brown joined the Seahawks as a free agent in 1997, a year after registering 13 sacks with the Pittsburgh Steelers.
Brown, 32, led the Seahawks in tackles in his first three seasons. He has started all 84 games he's played since joining the Seahawks, missing a game in 1997 and 1999 with injuries.
In 142 career games - 135 starts - Brown has 71 sacks.