The NFL season changed dramatically yesterday when the New England Patriots appeared to suffer the most serious setback imaginable: Quarterback Tom Brady is expected to miss the rest of the year after a serious knee injury on the NFL’s opening Sunday.

Although the full extent of Brady’s injury won’t be known until an MRI exam is conducted, reports last night indicated that he had sustained significant ligament damage and wouldn’t be able to play again this season. The absence of Brady would have ramifications so far-reaching that his injury could be the most significant in the history of the league.

The immediate ramifications are that the Patriots look like a team in turmoil, eight months after they completed the first 16-0 regular season in NFL history and 24 hours after they opened the 2008 season as the Super Bowl favorites. New England’s second-string quarterback is Matt Cassel, a player who not only hasn’t started an NFL game, but never even started a college game — he was strictly a backup, behind Heisman Trophy winners Carson Palmer and Matt Leinart during his tenure at USC.

Cassel turned in a solid effort yesterday, completing 13 of 18 passes for 152 yards with a touchdown and no interceptions as the Patriots beat the Chiefs, 17-10. But no matter how many solid efforts he turns in, he’s no Tom Brady.

That’s because there is no other Tom Brady, who last year had what many consider the greatest statistical season any quarterback has ever had. After Brady won the league’s Most Valuable Player award and threw an NFL-record 50 touchdown passes, no one could expect Cassel to give the Patriots even close to the production that Brady provided last year.

And that means no one can think the Patriots are the favorites to win the Super Bowl. In fact, it’s hard to see how they’re even favorites to make the playoffs without Brady. Perhaps that’s why Brady’s favorite receiver, Randy Moss, looked so crestfallen after yesterday’s game, as he claimed that Kansas City Chiefs safety Bernard Pollard was a dirty player for his hit on Brady.

It’s understandable that Moss was emotional, but he was wrong about Pollard, who was not penalized on the play, in which his shoulder collided with Brady’s left leg just as Brady released his pass. The NFL is not expected to fine Pollard, and it shouldn’t: The hit was unfortunate, but it wasn’t dirty.

The loss of a big-name player at the start of the season is not without precedent. In 1991, Philadelphia Eagles quarterback Randall Cunningham, a major star coming off three straight Pro Bowl seasons, suffered a season-ending knee injury in Week 1. In 1997, San Francisco 49ers wide receiver Jerry Rice, at the time one of the league’s best players, tore his ACL in the first game. And in 1999, Jets quarterback Vinny Testaverde, coming off the best season of his career, suffered a ruptured Achilles tendon that ended his season and crushed the Jets.

But while all of those players were significant, none meant as much to the league as Brady. Brady is the unquestioned leader and best player on the NFL’s best team, and he’s even more than that. He’s one of the best players in football history, on one of the best teams in football history.

Now, with Cassel at the helm for the foreseeable future, the Patriots are just another team, perhaps not even the favorites to win the AFC East. The Jets and Buffalo Bills both won their openers yesterday, and the division that the Patriots have dominated in recent years now looks very competitive.

The Patriots may go shopping for a veteran quarterback to back up Cassel, and if they do, the logical player to approach would be Testaverde, who backed Brady up in 2006 and started six games for the Carolina Panthers last season but is currently unemployed. Testaverde is an old pro who can be ready at a moment’s notice, and Cassel may turn out to be a better quarterback than anyone realizes. But neither can possibly do for the Patriots — or the league — what Brady has done during the last seven seasons.