FOXBORO - At around 1:20 p.m. Eastern Standard Time on Sunday, the collective nightmare of Patriots Nation became reality.

Tom Brady was on the turf at Gillette Stadium, clutching his left knee, rolling around in obvious pain.

As they say in text parlance, OMG.

Brady's first-quarter injury, suffered as he was completing a pass downfield to Randy Moss, cast a pall over New England's opening day 17-10 win over the Kansas City Chiefs.

With just 7:33 gone in the 2008 season, Sammy Morris did a great job taking out Chiefs safety Bernard Pollard in blitz pickup. But Brady hadn't let go of the ball yet, so Pollard got up on his knees and dove for Brady's legs -- with Morris still draped on him -- and his helmet hit square on the outside of Brady's knee.

The reigning league MVP crumpled to the turf.

And the course of the Patriots' season may have been irretrievably altered.

Brady was able to walk off the field on his own, though with a limp, and was able to walk down the stairs leading to locker room tunnel without assistance. After the game, Bill Belichick said there was no update on Brady's status.

"They are looking at him, doing some tests on him, so we'll see how that turns out," he said, before joking, "I doubt anybody is interested in that, but just in case you were going to ask."

Perhaps Belichick's moment of levity is a sign that things aren't so bad. Or it could be a smoke screen.

And Pollard, a third-year strong safety who was Kansas City's second-round draft choice in 2006, was left to defend his play.

"The play was not intentional," he said. "People can call me a dirty player, you can call me whatever you want to call me; it's not a dirty play. I saw the ball was still in his hands and I tried to get to him. I tried to get up and get to him. But I couldn't get up, so I just tried to grab him. It was not an intentional play."

Pollard added that he heard Brady "scream and yell," and that's when he knew something was wrong. The Indiana native said he told coach Herm Edwards that he wasn't trying to hurt Brady.

In Brady's stead came Matt Cassel, who acquitted himself fairly well, particularly on his first drive. Starting on his own 2-yard line, Cassel was facing third and 11 and unloaded a bomb for Randy Moss down the right side. It was a 51-yard pickup, the longest pass of Cassel's career.

A few plays later, Cassel rifled a wobbly pass into the back of the end zone, a little high, but not at all too high for the 6-foot-4 Moss to haul in for his first score of the season and the 126th touchdown of his career.

Cassel and the offense struggled a bit on New England's next two possessions, going three and out on each. But he led the team into the end zone again on his first drive of the second half.

In all, Cassel was 13 for 18 for 152 yards with a touchdown and two sacks. In 17 preseason drives, he did not lead the Pats into the end zone, leading to speculation that he wouldn't be kept on as Brady's backup.

"It was good," Belichick said of Cassel's play. "It could have been better, but I thought he handled the team well and some accurate throws. That was a big play [to Moss] coming off the goal line. He managed the team well." On the other side of the ball, the Pats' offense had some high notes -- a goal-line stand at the end of the game to seal the win, four sacks and an Ellis Hobbs interception -- and one or two low notes, as they allowed the Chiefs to convert half of their third-down chances and one of their two red-zone opportunities.

The first sack, by Adalius Thomas early in the third quarter, knocked starter Brodie Croyle out of the game with a separated shoulder.

Cornerback Deltha O'Neal, signed Monday, made two plays during Kansas City's final possession, first tracking receiver Devard Darling down from behind and keeping him out of the end zone after a 68-yard catch-and-run, and then preventing Dwayne Bowe from getting backup (and former Patriot) Damon Huard's last-chance touchdown try.

Both O'Neal and Bowe were motioning for pass interference on the play; no flag was thrown.

"I just slapped his arm before he caught the ball. I believe it's legal; they didn't call nothing," O'Neal said when asked about the play with Bowe. "It felt good [to make plays]. It felt like I'm contributing."

Unfortunately for O'Neal, Hobbs and the rest of the Patriots, their play -- and their season -- may be overshadowed by Brady's knee.