Since 2000, five of Bill Belichick's assistant coaches have become head coaches in the NFL.

Romeo Crennel was coach of the Cleveland Browns after winning three championships as defensive coordinator of the New England Patriots. Crennel went 24-40 as coach of the Browns.

Eric Mangini succeeded Crennel with similar results: a 10-22 record that led to his firing.

Josh McDaniels coached the the Denver Broncos for two years before getting fired this year. He went 11-17, and 5-17 since a 6-0 start in 2009.

Charlie Weis coached Notre Dame and for the first two years led the Fighting Irish to the Fiesta Bowl and the Sugar Bowl. But then hard times fell, as Weis and the Fighting Irish went 16-21 the next three seasons, subsequently leading to Weis' firing in 2009.

Nick Saban coached the Miami Dolphins for two years in 2005 and 2006. He went 15-17.

See a pattern here?

In fact, the only understudy of Belichick's that still has a head coaching job in the NFL is Jim Schwartz, who coached under Belichick with the Browns in the mid-1990s and now is head coach of the Detroit Lions.

How have so many men fallen after leaving Belichick's teams?

Here are 10 reasons.

10. They Try to Copy Belichick's Style

It's only natural to try to mimic a mentor's style in order to grow.

But there's a difference between mimicking and copying someone's style.

Hell, it's even gone as far as Josh McDaniels creating a "Spygate II" after illegally videotaping a 49ers walkthrough practice during the Broncos-49ers game in London this year.

It would be foolish not to take some things from Belichick and utilize them, but you can't be Bill Belichick. Every coach has to have his own style, his own way of succeeding. You can't be someone you're not.

9. They Coach Bad Football Teams

Romeo Crennel and Eric Mangini had the pleasure of coaching none other than the Cleveland Browns.

Josh McDaniels coached the Denver Broncos.

None of these teams are powerhouses.

People forget that in Belichick's five seasons as head coach of the Browns from 1991-1995, he went 36-44. That's not even a winning record.

Fans expect Belichick's understudies to revitalize a team miraculously despite having little talent because they worked under Belichick. But Belichick didn't exactly succeed with a bad Browns team himself.

Which brings me to my next point...

8. The Expectations Are Too High

You can say these coaches failed, and in some respects you would be right.

But you also have to be realistic.

It takes talent to succeed, pure and simple. Team chemistry is important. Being on the same page is important. Execution is important. But you need talented players to win, that's the bottom line.

Even a very good coach could fail under the circumstances these coaches were put in.

7. They Don't Know Talent As Well

Of course, even when you have no talent, you can bring it in eventually.

Belichick is undoubtedly a great evaluator of talent.

Tom Brady turned out to be golden, and Matt Cassel turned out to be quite the backup as well.

You can even look as recently as this season, with the undrafted BenJarvus Green-Ellis leading the Patriots' rushing attack, along with New York Jets castoff Danny Woodhead.

One of McDaniels' biggest criticisms is 40 of the 53 men on the Broncos' current roster were chosen by him. It's safe to say McDaniels was not the best evaluator of talent.

6. They're Too Stubborn For Their Own Good

Bill Belichick is a stubborn man.

There's no question about that.

He sticks to his guns and seems to be immune to any influences, whether it be fans or the media or even his own players.

But Belichick has earned the right to be this stubborn, because he's proven he knows how to win.

Being stubborn and determined can be a good trait, but if you aren't winning, then it can be a hindrance.

Sometimes you have to change your thinking and adjust.

McDaniels, Mangini, and Weis were all accused of arrogance during their tenures, and maybe deservedly so.

5. Belichick Knows the Game Better

Bill Belichick has been called a genius, a mastermind, even a mad genius.

He knows football as if it's been ingrained in his head since birth.

Sure, Belichick's assistant coaches can learn under him, pick up on things, but you also have to learn for yourself through trial and error.

Belichick seems to know exactly how to fix deficiencies, and I've got to think this comes from years being in the game.

Remember when the Patriots pass defense was considered really bad early in the season? Remember when they lost 28-14 to the New York Jets and lost to the Cleveland Browns?

That seems like it was ages ago. Those problems are no longer problems for the Patriots anymore.

4. Belichick Makes It Look Too Easy

It must be hard to acclimate to the harsh reality of head coaching in the NFL after being nestled warmly under Bill Belichick's flawless regime.

Kinda like when your parents take your training wheels off your bike as a child and tell you to go ahead and continue as if the training wheels never came off.

You end up falling a lot and ultimately hurting yourself, and you wonder why those training wheels ever had to come off in the first place.

Training under Bill Belichick is like training under Lance Armstrong; you get the idea, you know to a degree what it takes to succeed, but you can never dominate the way he does no matter how hard to try.

3. They Didn't Learn Under Bill Parcells

People forget that before Bill Belichick began succeeding as a head coach with the Patriots, the legendary Bill Parcells was his mentor with the New York Jets.

Check out the former Parcells assistants compared to Belichick's: Bill Belichick, Sean Payton, Tony Sparano, Tom Coughlin, and Todd Haley.

By the looks of things, Parcells was at least a better teacher to his coaches than Belichick was.

2. They Aren't Bill Belichick

Bill Belichick is one of the greatest coaches the game has ever seen.

Right now, it's hard to find anyone that even compares to him.

No matter how hard you try, you can't be Belichick.

In order to turn some of these teams around, Belichick's former assistants basically had to be masterminds.

They are not, and few coaches are even close.

1. They Don't Have Tom Brady

Funny how Bill Belichick's only losing season with the Patriots was the 2000-2001 season, when Tom Brady was a rookie and had only three pass attempts all year.

Belichick went 5-11 that year.

Enter Brady as the starter in 2001. Belichick never had less than nine wins the rest of the way.

It's no secret you have to have a good quarterback to succeed in the NFL. Tom Brady is arguably the best quarterback in the game today.

No other coach has Tom Brady.