The following group of under-performing coaches and players are making many experts and pundits and even the most optimistic fans look foolish. 

Dave Wannstedt and Mario Williams, Buffalo Bills

When the Buffalo Bills promoted Dave Wannstedt to coordinator and gave him Mario Williams in free agency, Bills fans must have thought they had finally found the right formula for a strong defense.

Five games into the 2012 season, not only have those expectations been dampened, the Bills defense is worse than ever.

Wannstedt's 4-3 front surrendered 621 yards of total offense to the San Francisco 49ers in Week 5, one week after allowing the New England Patriots to pile up 580 yards.

The Bills are 31st in yards allowed and scoring defense.

Mario Williams has simply not delivered for the Buffalo Bills' ailing defense.
Timothy T. Ludwig-US PRESSWIRE

Part of the problem has been the lack of production from prized free-agent rush end Williams. The Bills thought they had landed a gem when they won a lengthy audition for the 2006 first overall pick's services.

However, things have not gone to plan for Williams, who has just 11 tackles and 1.5 sacks so far this season. Not many saw this coming. 

Romeo Crennel, Kansas City Chiefs

Romeo Crennel's appointment was supposed to cure all of the ills of the Todd Haley regime and bring the Kansas City Chiefs back to the level they played at in 2010.

Instead, Crennel's team has only one win to their credit.

The biggest surprise is how poorly the defense started the season, following their former coordinator's promotion to the top job. They have improved in recent weeks, but their 28th ranking in points shows how much Crennel's defense has struggled early.

Rather than improve from last season, the Kansas City Chiefs have continued to regress under Romeo Crennel.
John David Mercer-US PRESSWIRE

Of course, they haven't been helped by an inconsistent offense that still can't find a passing game to complement the efforts of star running back Jamaal Charles.

The Chiefs were considered a contender before the season began. The real surprise will now be if Crennel's team can climb out of the AFC West basement before there are any more major changes. 

Ryan Mathews, San Diego Chargers

Having made a quick return from a clavicle injury suffered just prior to the season, San Diego Chargers running back Ryan Mathews found himself under immediate pressure.

He lost a key fumble against the Atlanta Falcons in Week 3, which prompted general manager A.J. Smith to warn that Mathews might have to find a new team if his ball security doesn't improve.

Add RB Ryan Mathews to the list of talented, but under-performing San Diego Chargers stars.
Jake Roth-US PRESSWIRE

This was supposed to be a season where Mathews confirmed his status as a rising star at the running back position. He enjoyed a breakout campaign in 2011, rushing for over 1,000 yards for the first time as a pro.

However, more injury problems and continued issues protecting the ball mean Mathews is already heading for an important crossroads in his NFL career. 

Ron Rivera and Cam Newton, Carolina Panthers

The Carolina Panthers seemed certain to build on their impressive statistical performances from 2011. That confidence came from the belief that young quarterback Cam Newton would continue to terrorise every defense he encountered, while defensive-minded head coach Ron Rivera would turn around a soft unit.

Neither has really happened for the Panthers. The biggest surprise has to be just how much Newton and the offense have struggled. Things started badly when the Tampa Bay Buccaneers managed to hold a ground game featuring Newton and DeAngelo Williams to a puny 10 yards in Week 1.

Cam Newton hasn't been able to dominate NFL defenses the way he did as a rookie.
Jeremy Brevard-US PRESSWIRE

Since then, both the New York Giants and Seattle Seahawks have dominated Newton and company in Carolina. Oddly enough, these issues have undermined some slight improvements defensively.

The front seven is generating a decent pass rush and the unit as a whole is tougher in the red zone. However, the spotlight remains firmly on Newton and if he continues to regress from the heights of his record-setting rookie season, Rivera could pay a heavy price.