FOXBORO -- The Patriots season changed dramatically Sunday during a surprisingly difficult 17-10 victory over the Kansas City Chiefs, but it did not end. If any team in pro football understands that it should be the Patriots, who six years, 11 months and 16 days ago suffered what appeared to be nearly as catastrophic a loss as they did against the Chiefs when Tom Brady’s left knee went one way while his leg went the other and his ligaments protested.

Hours later, NFL sources close to the situation say that Brady experienced a torn ACL in his left knee.

Brady was down for quite some time before leaving and not coming back. In his stead a backup nearly as unknown as Brady was in 2001 when Drew Bledsoe’s insides exploded after a hit from Mo Lewis, trotted onto the field and almost immediately led them on a critical 10 play, 98-yard touchdown drive that included a 51-yard completion on a sight adjustment on third-and-11 from their own 1 and a 10-yard touchdown strike on third-and-8 at the Chiefs’ 10 that give New England a lead it fought valiantly all afternoon to maintain.

That is not to say this is dawning of the Matt Cassel Era in New England or the end of Tom Brady’s time as the sport’s premier quarterback, although from the looks and sound of it you won’t be seeing the latter next weekend in a much-anticipated dual with Brett Favre nor a week later against the Dolphins either because sources are already whispering the most dreaded letters in sports – torn ACL.

Even if the injury proves less severe, if you see Brady at all this season it may not be until the shadows have grown long and the sun dim. The larger point is, for the moment at least, that the Patriots’ own history tells them that even the loss of their long-time leader doesn’t necessarily mean all else is lost. That is something only time and Matt Cassel will make clear.

“Injuries have affected us in the past and whatever it is we do a good job focusing on what we need to do,’’ cornerback Ellis Hobbs said. “One person does not run this whole team, Tom will tell you that, anybody will tell you that. Bill has told us that. One person does not make this team. We have guys who play big roles and do their job, but at the same time the season must go on and whoever steps in for him, as you saw today, has to do their job. That's why you’re a professional, that's why you get paid. You wouldn't be here if you couldn't do it.

“I wasn't in the NFL yet, but I remember a guy who got a chance a couple of years ago like that, who took it all the way and the rest is history.’’

For this season Tom Brady may be as well but no one truly knows yet. A few weeks ago, you may recall, New York Giants’ sack artist Osi Umenyiora went down and was declared by Tom Coughlin to be not seriously hurt. A day later he was out for the season. Things change quickly in the NFL, as they did Sunday at 7:32 of the first quarter, when Chiefs’ safety Bernard Pollard grabbed at Brady’s firmly planted left leg and sent his knee in a direction medical science would not advise it to go. After he left the field, wide receiver Randy Moss admitted he kept looking for him to return. When he did not, he did what great receivers do. He adjusted, although such things take a little time.

“After he went to the locker room, every time the fans cheered I looked over at the (locker room) door," Moss said. “I was like a kid at the candy store just hoping you would see that No. 12 come out those doors and up the steps.

“Every time the fans cheered I looked at that door so basically I was just snapping my neck around every time. I don’t know his injury or what happened but all I know is he didn’t come back. Matt Cassel had a hell of a game and hopefully next week he can pick up where he left off."

Cassel‘s play was up and down but more up than down when it counted. He ended with a 116 quarterback rating, led three key scoring drives at the most important junctures of the game and left with a win. He also did it against one of the youngest and least talented teams in the league but, again, so what? He did it, which is far better than the alternative.

In the NFL there is nothing more important than making the plays at the critical moments, of which there are seldom more than two or three a game. Not stats or commercials or high profile girlfriends. What is important is that you make the plays that win games and in his first extended playing time since high school, Cassel did.

He was on the same page as Moss on the 51-yard throw that got his team out of the shadow of its own goal line on his first drive of the day. He threw a perfect ball to Moss to end that drive with a touchdown. He led the Patriots on a second scoring drive to open the second half, giving New England a 14-3 lead and reminding the upstart Chiefs that while they may have knocked out New England’s rock star it would be more difficult to knock his team off.

Finally, with the score now 14-10 and Hobbs having just picked off a pass from Damon Huard midway through the fourth quarter, Cassel directed a crisply executed 10 play drive that produced a field goal to push New England’s advantage back to seven with 2:26 to play, meaning the Chiefs needed a touchdown just to force overtime and had only 140 seconds to do it. They could not, although they came closer than anyone in New England would have liked.

“He played better than I thought he was going to play,’’ admitted Chiefs’ linebacker Derrick Johnson ruefully. “He played realy good. He held up. He held his own."

Whether he can continue to do that in the weeks ahead not even Pete Sheppard knows. Neither do the Patriots, just as they didn’t know six years, 11 months and 16 days ago what would happen when an unknown sixth round-draft choice named Tom Brady first came onto the field. What they did know though was that he had been a key part of a solid University of Michigan team so they could at least be sure he could line the offense up while Cassel hasn’t started a game since he was a senior at Chatsworth High School in Southern California during the 1999 season.

But who’s counting? If that worries you consider this: Bill Belichick’s coaching record is 41-56 with any starting quarterback not named Tom Brady. That’s 15 games below .500 to help you with the math but, again, so what? He didn’t have Moss and Richard Seymour and Rodney Harrison and Vince Wilfork and Ty Warren and Mike Vrabel and Adalius Thomas or much of anything else going for him in those days.

Will any of these things matter next Sunday against the Jets in the Meadowlands? No they won’t, just as they didn’t matter after Brady limped off the field with men supporting him on either side. All that will matter is how Cassel plays and, perhaps more importantly, how the people around him play.

“This is something I’ve been preparing for for a long time," Cassel said after completing 13 of 18 for 152 yards and one score with no interceptions. “You never wish injury on anybody. Since I’ve been around Tom, he’s always popped back up. I didn’t know what was going to go on. I just buckled my chinstrap and if that was what was going to be, then that’s what it was going to be.

“As a quarterback you have to deal with adversity and I think through adversity comes advancement. You start to settle in a little bit and like I said I have to rely on those guys and those guys rallied around me. I felt their support, both offensively and defensively. You just have to go out there and focus on your job and continue to execute from one series to the next.

“There was a huge emotional charge after the game. I’ve been working for this for a long time. It was definitely rewarding today to get that victory, there’s no doubt about that."

No doubt at all. The doubt is what happens next.

Ron Borges is a regular contributor for WEEI.com. He can be reached at rborges@weei.com.