updated 11:05 p.m. ET Sept. 7, 2008

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.

Multiple media outlets, including Yahoo! Sports and ESPN later reported that Brady suffered a torn ACL to knock him out for the year, and Peter King of NBC Sports reported that the injury was "serious."

“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.

Various reports Sunday night said Belichick told the players that the injury was serious. The Patriots were already looking for a possible replacement for Brady. A person familiar with the situation told The Associated Press that former Tampa Bay quarterback Chris Simms was coming in Monday for a physical.

“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.

Video   SNF Roundtable: Week 1 Sept. 7: The Football Night in America crew debates whether or not the hit on Tom Brady was dirty and discusses the implications of the QB's injury for the NFL. NBC Sports

“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.”

Not this time. And 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.”

Special feature NFL's opening act A look at the top photos from Week 1, including a beat down in Oakland and more. NBCSports.com

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.”

But, Chiefs cornerback Patrick Surtain said, “Brady is Brady. There’s only one of him.”

Last season, Brady set an NFL record with 50 touchdown passes, but it ended in disappointment with the Super Bowl loss.

They stayed calm without him Sunday.

“Worry about the consequences later,” linebacker Tedy Bruschi said. “Whoever’s in there, do your job.”

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