Bill Belichick has experienced this nightmare before. Three times, in fact. Two ended with perhaps his proudest moments in coaching.

He was the Giants’ defensive coordinator in 1990, when starting quarterback Phil Simms suffered a season-ending foot injury in December.

He held the same post with the Jets in 1999, when Vinny Testaverde’s Achilles’ tendon gave out in the second quarter of the first game.

He was the Patriots’ head coach in 2001, when a truck named Mo Lewis hit Drew Bledsoe so hard that it sheared a blood vessel in his chest and put him out for three months.

The first and third scenarios ended with Super Bowl championships, so if anybody thinks Tom Brady’s season-ending knee injury is a death sentence for the Patriots, well, you need to start paying more attention.

The Patriots aren’t done yet. Obviously, they’re no longer the Super Bowl favorite in the AFC — let’s be honest here — but only a fool would dismiss them entirely. They’re too well-coached and too talented to curl up and quit on the season. Belichick, leaning on his past experiences, will make sure that doesn’t happen with Matt Cassel at quarterback.

“(Brady) played one position. He played it well. There will be someone else playing that position now,” Belichick said matter-of-factly — a cold, but necessary response to the devastating news.

There’s no crying in football. Only regrouping.

What Belichick learned from the Simms situation was how to adjust on the fly. The Giants’ longtime backup, Jeff Hostetler, was more mobile than Simms, so the coaches tweaked their offensive philosophy, incorporating bootlegs and sprint outs in the game plans. They played to Hostetler’s strengths and became more of a bruising, ball-control team, and that paid huge dividends in Super Bowl XXV, where they absolutely pulverized the heavily-favored Bills.

What he learned from the Testaverde debacle was the importance of keeping the faith. When Testaverde went down, the Jets, a trendy Super Bowl pick, slipped into a deep funk. Even the tough-guy coach, Bill Parcells, checked out emotionally.

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The team began 0-4, but they caught a spark from a neophyte quarterback named Ray Lucas. They rallied to 8-8, narrowly missing the playoffs. It ended up being one of the best coaching jobs in Parcells’ career, but it left a painful what-if question: What if they had responded better in the immediate aftermath of the injury?

What Belichick learned from the Bledsoe injury was this: You never know. They inserted an untested Brady, scaled back the offense and rode a tough, intelligent defense all the way to a stunning win over t he Rams in Super Bowl XXXVI.

The Patriots’ current plight is unlike any of the aforementioned three. In terms of pure shock, it resembles Testaverde and the Jets. In 2001, there wasn’t the same sense of doom with Bledsoe because they didn’t know if he’d be gone for the season. With Brady, the reigning league MVP, the Patriots know. Psychologically, the team has to deal with that, but Belichick, whose at his best when dealing with potential distractions (see: SpyGate), won’t let his team wallow in self pity.

In some ways, the Patriots are better equipped to handle this than they were in 2001. They have a deep backfield and a solid offensive line, so they will shift gears and become more of a run-oriented offense. They can’t forget about wide receiver Randy Moss, but the coaches will have to change his route package. Those 50-yard ‘go’ routes he ran with Brady? Forget it; he’ll have to become more of a possession receiver with Cassel, who doesn’t have Brady’s arm.

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Defensively, the Patriots have enough talent in the front seven to control and win games. They will struggle against potent passing attacks — Belichick can’t feel too comfy with his thin secondary — but aside from the Colts, how many dangerous passing teams reside in the AFC? Not many.

Belichick’s problem is that Cassel is no Tom Brady. True, Brady was virtually untested when he replaced Bledsoe, but at least he had a resume as a winning quarterback at Michigan, where he played in big games and bowl games.

Cassel? We don’t know anything about him. He carried the clipboard at Southern Cal (yes, you can get a scholarship for that), backing up Heisman Trophy winners Carson Palmer and Matt Leinart. He attempted only 33 passes in college and threw only 39 before stepping in for Brady Sunday against the Chiefs. His arm is strong enough, but let’s not go wild with the Brady comparisons, please: Cassel was mediocre in the preseason and n early got cut.

Hannah Foslien / Getty Images The Week in Sports Pictures

But the Patriots will find a way to adjust and stay competitive, and — and no, wise guy, that doesn’t mean more video cameras. Their foundation is too strong to be destroyed by one bad storm.

“He didn’t make that team,” said guard Brandon Moore of the Jets, who now have Brett Favre and a clean shot at the division title. “The team made that team.”

Brady is an all-time great, and his loss probably will cost the Patriots five or six wins, but they’re still an eight- or nine-win team. They don’t own a white flag.