FOXBOROUGH, Mass. — Tom Brady screamed. The fans went quiet.

The seemingly indestructible star of the New England Patriots lay on the ground, clutching his left knee. The NFL's reigning MVP and three-time Super Bowl champion was done for the day — at least.

"He was in a lot of pain. When you hear a scream, you know that," Kansas City safety Bernard Pollard said after hitting Brady on the left leg midway through the first quarter.

Untested Matt Cassel took over, and played well as the Patriots beat the Chiefs, 17-10, in yesterday's opener.

There was no official word on the extent of Brady's injury — "I am not sure what we are dealing with," coach Bill Belichick said — or how long he would be out. But his 128-game starting streak, third-longest in NFL history, is in jeopardy.

"It kind of looked bad," Randy Moss said. "I know the show must go on and, hopefully, Matt Cassel is ready to step in."

Some media outlets are reporting Brady will miss the rest of the season due to a serious knee injury.

Cassel, who had thrown just 39 passes in his first three seasons, went 13 for 18 for 152 yards and one touchdown yesterday.

"This is something I've been preparing for (for) a long time. It's not something that we expected to come up on opening day," he said. "Since I've been here and been around Tom, he's always popped back up."

Brady finished 7 of 11 for 76 yards.