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FOXBORO - For some time now, the law of averages has been lurking quietly in the shadows of Gillette Stadium, waiting for its chance to strike.

The talented quarterback of the NFL's only true dynasty of the New Millennium had started 127 straight games, missing only one half of one game due to injury, and that was seven years ago. As much as Patriot Nation wished that their leader could remain invulnerable forever, the odds were against it.

With 7:27 left in the first quarter Sunday, the worst fears of a franchise were realized. Grasping his left knee and writhing in pain on the Gillette Stadium FieldTurf after being undermined by Kansas City safety Bernard Pollard, three-time Super Bowl winner Tom Brady was done for the day and for who knows how long after that.

That the Patriots won the game, 17-10, isn't really important.

What's important is how long Brady will be sidelined - something to which reporters and fans alike can only speculate, given the code of silence followed by everyone within the Patriots' organization - and whether the oft-criticized understudy, Matt Cassel, can keep the ship afloat until such a time that Brady is ready to return. This is not 2001, when a fresh-faced Brady replaced a declining Drew Bledsoe and breathed life into an offense that had become as stale and stodgy as its injured ex-leader. The Patriots were ready for Brady in 2001, even if he wasn't quite ready for them. Seven years later, the prevailing notion is that the Patriots aren't ready to venture ahead without Brady for a considerable length of time.

But games are played on the field, and not in notions. Cassel, the former Southern Cal backup whose last appreciable play behind center came as a student at Chatsworth (Calif.) High School, knew that at some point in his professional career, he might be called upon to lead. It hadn't happened in three years, but it happened just seven minutes into the first game following what had seemed to many to be a preseason of incompetence that would have warranted his release.

"This is something I've been preparing for for a long time," he said. "It's not something we expected to come up on opening day ... I just buckled my chin strap and if that was what was going to be, that that's what it was going to be,"

As if to compound matters, he got the ball for the first time at his own 2-yard line. The first two carries netted a loss of a yard, and a smattering of boos could be heard from what had been an otherwise shocked-into-silence Gillette Stadium crowd.

Cassel's fortunes turned on the next snap.

Standing tall in the pocket on the last play of the first quarter, he fired a bullet to Randy Moss for a 51-yard gain in front of the Patriots' bench.

"That was a big play coming off the goal line where he hit Moss," Patriots' coach Bill Belichick said. "That was right in front of me ... I got a good look at that one. He made a great throw on that, coming out of the end zone."

Getting some help from running back Sammy Morris, who broke off a 22-yard scamper to get into the Chiefs' red zone, Cassel threw over the middle to Moss on a third-and-8 play for a 10-yard touchdown play with 11:45 left in the first half. The drive covered 98 yards in 10 plays and served, at least for the time being, to reinforce the Patriots' confidence that they can compete and win regardless of the obstacles put in their way.

"I thought he did a pretty good job coming in," offensive tackle Matt Light said. "Obviously he made some big plays for us, and it's nice when that happens. That guy's had a lot of criticism cast down on him, and I feel like he stepped up and played like a professional."

"I let him know that we have his back," wide receiver Wes Welker said, "and we're going to go out there and do the best we can for him, and try to make plays and make his job as easy as possible. I think he did pretty good for a guy who you all thought should have been cut." Cassel finished the day completing 13 of 18 passes for 152 yards and one touchdown, for a 116.0 passer rating. He never yielded the lead, and much like the Brady of 2001, tried not to overextend himself or attempt to do more than just execute the offense as it was scripted for him.

"When guys go down around here, you just anticipate that the next person in line is prepared, and we know that he did his work, so let's keep playing," linebacker Tedy Bruschi said. "We worry about the consequences later. It's whoever's in there, get the job done."

Amid the bravado, at least one Patriot who has benefitted by Brady's presence allowed himself a few moments of honesty in regard to his quarterback's sudden absence and uncertain future.

"He's the face of the New England Patriots, and Tom being who he is and the competitor that he is, it kinda hurts, to be honest with you," said Moss, whose six catches for 116 yards helped smooth the transition. "I know the show must go on. Matt Cassel is ready to step in, and we have to embrace him and let him lead us.

"But every time the fans started cheering, I'd look at that door," he said, referring to the runway to the locker room into which Brady had disappeared to have his left knee examined. "I was like a little kid at the candy shop, just hoping you would see that No. 12 come out those doors and up those steps."

He wasn't alone.

MARK FARINELLA may be reached at 508-236-0315 or via e-mail at mfarinel@thesunchronicle.com. Read Farinella's blog, "Blogging Fearlessly," at thesunchronicle.com/farinella.