New England Patriots quarterback Tom Brady is expected to miss the rest of the 2008 season with what two sources described as a severe injury to his left knee, suffered in the first quarter of the Pats 17-10 victory over the Kansas City Chiefs at Gillette Stadium Sunday.

While New England coach Bill Belichick said in his postgame press conference that he had no update on the status of Bradys injury, a source close to the quarterback indicated it involved serious damage to the knee. That could include a torn anterior cruciate ligament and perhaps other ligament and cartilage damage.

Its bad, a team source said. Were going to have to play without him.

Brady was hurt as he stepped up in the pocket to make a throw to wideout Randy Moss. Chiefs safety Bernard Pollard, who was on the ground, lunged forward and struck Bradys leg, which buckled as he released the ball.

Brady was helped off the field and was later led down a staircase by team officials, presumably to undergo tests.

Fourth-year quarterback Matt Cassel, who had thrown just 39 passes in his career before Sunday, replaced Brady and completed 13 of 18 passes for 152 yards and a touchdown. Cassel, who backed up Carson Palmer and Matt Leinart at USC, hasnt been a starting quarterback since high school. He won a competition with Matt Gutierrez during the preseason to retain the job as Bradys primary backup.