Everybody was happy to see a relatively healthy Tom Brady take the field in Foxborough, Mass., to start the New England Patriots' season opener against the Kansas City Chiefs. That joy was short-lived.

At the halfway point of the first quarter (7:27 left), while completing a 26-yard pass to Randy Moss, Brady took a low blow from Chiefs safety Bernard Pollard in the pocket, injuring his left knee. Unfortunately, the news is as bad as it gets for New England: Brady, whose injury is unrelated to the right foot that bothered him during the offseason, is reportedly out for the rest of the 2008 season.

And with that, both the Patriots and every pro football fan on the planet will find out why he is the game's most irreplaceable player.

First, whatever happens during the NFL's 08 season will just seem empty without the game's most exemplary performer. It will have the feel of a watered-down year, because the game's best isn't out there. The closest feeling to this in my time as a sports fan is when Michael Jordan took two long years off between championship runs.

We've been fortunate to see Brady already enjoy the ultimate team success with three Super Bowl rings and then reach individual heights last season. But that only makes us feel now that we're being unfairly robbed of seeing something special for the next five months.

Brady's face is all over magazines and celebrity websites, but despite his all-American status a sex symbol and fashion icon, we know the one who straps on the pads and fires downfield lasers is the real him. We've seen a lot of Brady without his helmet but not like this, on the sideline where he can't help his team.

Then of course, are the obvious ramifications on the field. On Sunday, the Patriots survived a final Kansas City threat inside the 10 to hold on for a 17-10 win as backup Matt Cassel (13-of-18, 152 yards, one touchdown) turned in an efficient performance. Cassel did just what was required, as the running game (Sammy Morris and Laurence Maroney), defense (sacks and takeaways) and special teams (a big return) all helped.

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Even without Brady, who has started 128 consecutive games, the team is talented enough to win most of its games -- New England does have the NFL's weakest schedule -- with that kind of team effort. But Brady's emotions, energy and passing skills will be sorely missed in tougher, high-intensity matchups against teams better than the Chiefs, losers of 10 consecutive games.

One of those games comes immediately -- at Brett Favre's Jets next Sunday. Cassel will need to step into a hostile road environment where the pressure is on the gain a big win, a situation in which Brady was unflappable.

"This is something I've been preparing for for a long time," Cassel said about getting extended playing time in case of injury to Brady. "It's not something I expected to come up on opening day."

You could see the lift Favre's exuberance gave the Jets. The same goes for Brady's fire -- it catches and spreads everywhere on the Patriots, where it's not good enough to just beat opponents, but light them up.

The Patriots were used to blowouts and high point-totals in winning every regular-season game last season. Now they'll need to go back to grinding out victories, much like they did on Sunday. So Cassel will need to develop Brady's trademark of playing his best in close big games, something that doesn't come with learning a playbook.

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Physically, it's a given that Brady can make some throws Cassel and most others can't. Emotionally, he gives everyone confidence -- not just his receivers, not just the rest of the offense, but also the bench and crowd. Even though Cassel was playing well and the Patriots were winning, there was an eerie calmness to Gillette Stadium for most of Sunday's game.

Brady has become like all elite quarterbacks to ever play in the NFL. He is much more than a passer and winner. His presence is so great on his team that it's difficult to even react to his absence.

"He's the face of the New England Patriots, and Tom being who he is it kind of hurts, to be honest with you," said Moss. "I know the show must go on. Hopefully Matt Cassel is ready to step in. I know the team is ready to embrace him and let him lead us."