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FOXBORO - You know today that Tom Brady is hurt.

Nobody knows how long he will be out of action.

This is when the philosophy of Bill Belichick kicks in.

"I don't know what Tom's circumstances are," Patriots' backup quarterback Matt Cassel said Sunday, "but I'm going to come in and approach it like I have each and every week. You never know when you're going to play and this is a great example of that."

Cassel, as unlikely a hero as can be imagined, replaced the injured Brady after the latter suffered a left knee injury with 7:27 left in the first quarter, completed his first pass for 51 yards to Randy Moss, and went on to lead the Patriots to a 17-10 victory over the Kansas City Chiefs in the season opener at Gillette Stadium. What could have been a plunge into the depths of football hell became a reprieve, for at least one Sunday, because the fourth-year veteran from Southern Cal never lost his cool and did simply what he was supposed to do - nothing more, nothing less.

Now, at least until anyone says differently, Cassel is the starting quarterback of the Patriots - a prospect that may not be as disconcerting as it seemed only a few short days ago.

"It's one of those situations where I'm going to prepare the same way, I'm going to go in there with the same energy and excitement and try and get ready for the Jets," he said.

Cassel completed 13 of 18 passes for 152 yards and one touchdown in relief of Brady, who was bowled over by Chiefs' safety Bernard Pollard after throwing a 28-yard completion to Moss (who fumbled it away) from his own 42.

The Cassel era started inauspiciously on the next Patriot possession (which started on his own 2), as two handoffs to Sammy Morris went for a loss of one yard. But on the last play of the first quarter, Cassel stood tall in the pocket and fired to Moss wide open on the right side for a gain of 51 yards to the Kansas City 48.

Cassel continued to work the passing game with throws to Morris and Kelley Washington, and Morris broke through the left side of the line for a gain of 22 yards to the Kansas City 12.

Then on third-and-8, Cassel fired to Moss on the back line of the end zone for a 10-yard score with 11:45 left in the half. It was Moss' 125th career TD reception (fourth all-time) and his 126th TD in all, tying Cleveland Browns' great Jim Brown for eighth place on the all-time list. The drive covered 98 yards in 10 plays and took 4:51 off the clock.

"I thought overall, he threw the ball pretty well," Belichick said. "He managed the team well. We didn't have a lot of misaligned plays and miscommunications. There is a lot of room for improvement, but I thought he stepped in and did a good job."

His teammates, fighting off their shock over fears that Brady may be down for the long term - and there were unconfirmed Internet reports that he had suffered a tear of the anterior cruciate ligament in his knee - offered similar praise for their new field general.

"I thought he played an outstanding game," said running back Laurence Maroney, who combined with Morris for 104 of the Patriots' 126 rushing yards. "He came in and stepped up and took control of the team. He had a great first play and we just took it from there." "He was able to remain calm," added Morris. "He's been playing hard and we have a lot of faith in him."

Indeed, Cassel managed the game well. He never lost the lead; Kansas City got a 40-yard field goal from Nick Novak with 17 seconds left in the first half to make it 7-3 at the break, then Cassel led the Patriots for 11 plays to cover 80 yards in the third quarter, leading to Morris' 5-yard touchdown run with 3:01 left. Passes of 18 and 13 yards to David Thomas and Wes Welker respectively moved the sticks, as did runs of 17 and 13 yards by Maroney and Morris.

Former Patriot backup Damon Huard, replacing injured starter Brodie Croyle (shoulder contusion), put the Chiefs on the board with a 13-yard touchdown pass to Dwayne Bowe four plays into the fourth quarter. But Huard was intercepted by Ellis Hobbs (victimized on that TD pass) two possessions later, and that set up a 10-play drive that put the Patriots up 17-10 on Stephen Gostkowski's 37-yard field goal with 2:26 left to play.

The Patriots needed to stage a goal-line stand when Huard (8-12, 118 yards, one TD, one interception) caught them in a blitz and completed a 68-yard pass to Devard Darling to the New England 5, newcomer Deltha O'Neal contributing a game-saving tackle. But the Chiefs never moved another inch, Huard missing connections with Bowe (covered by O'Neal) on fourth down.

That left the Patriots to ponder the future, possibly without Brady - although at least one of them wanted the NFL to ponder the hit by Pollard, as well.

"For me, personally, I think it was dirty," said Moss (six catches, 116 yards). "I didn't really see anything because I was running down the field. It looked dirty to me ... any time you see something like that that looks foul and looks dirty it opens your eyes. To me, personally, it looked dirty."

"Tom is definitely our leader," Maroney added, "but the team is the key factor. We need to stick together as a team and play together as team, which is the key to victory."

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.