Winslow Townson, Associated Press
New England Patriots QB Tom Brady (12) has his leg buckled by Kansas City Chiefs safety Bernard Pollard, bottom, on Sunday.

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 Sunday's opener. But they needed a last-minute defensive stand to do it.

There was no 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."

He was Sunday. That came as somewhat of a surprise after he failed to produce a touchdown in 17 exhibition series, leading many to wonder if he would even make the team on which he backed up Brady the past three seasons.

"He did a good job coming in and, obviously, made some big plays for us," tackle Matt Light said. "That guy's had a lot of criticism cast down on him, and I felt like he stepped up and played like a professional."

Still, the Patriots plummeted from an exclamation point of an unbeaten 2007 regular season to a huge question mark in 2008. They lost the Super Bowl to the New York Giants 17-14, then went 0-4 in the exhibition season while Brady had a right foot injury and missed all four games.

But for the first time in 57 games, Brady wasn't listed on the Patriots' injury report for Sunday's game. He completed seven of 11 passes for 76 yards.

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

"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."

Kansas City also had its own quarterback woes.

Damon Huard, playing after Brodie Croyle left with a bruised shoulder late in the third quarter, completed a 68-yard pass to Devard Darling, who cornerback Deltha O'Neal ran down and tackled at the Patriots' 5 with 53 seconds left.

"It was a gut check. Go get him," said O'Neal, a two-time Pro Bowler who signed early last week. "That's what my whole mentality was."

The Chiefs moved no further with two incompletions, a run for no gain by Larry Johnson and a poorly aimed pass toward Dwayne Bowe.

"Matt played well," Huard said. "At the end of the day, it's how quarterbacks are measured." -->

Chiefs cornerback Patrick Surtain said, "Brady is Brady. There's only one of him."