Sean Lee and Bruce Carter will determine the success of the Dallas Cowboys' new 4-3 defense. Even though the Cowboys have switched to a scheme featuring an extra defensive linemen, their two young linebackers will determine its success.

Wily old coordinator Monte Kiffin places a lot of coverage responsibility on his linebackers. They are charged with covering short, underneath areas and deep zones.

Kiffin demands superior athletes at the position. Fortunately that's exactly what he has in Lee and Carter.

If the pair stay healthy they are arguably the best tandem of young linebackers in the NFL. Both possess sideline-to-sideline speed and a knack for making plays in pass defense.

Kiffin's scheme will rely on their playmaking talents. Carter will have one of the most important roles as the weak-side linebacker.

In Kiffin's 'under' fronts, two defensive linemen are usually stacked over on the weakside. That keeps the weak-side 'backer covered up and free to make plays.

He must be the chief weapon in the underneath coverage schemes. Because Kiffin's Tampa-2 system forces quarterbacks to throw short, linebackers must quickly tackle receivers and keep gains minimal.

Carter certainly has the speed to make these plays. He'll also be challenged to disrupt any routes breaking across the middle, whenever the middle linebacker drops deep.

Having the middle linebacker rotate deep is the key element in Kiffin's coverage designs. It creates a three-deep look from a Cover-2 shell.

The middle linebacker will be Lee and he's ideally suited to dropping back between the safeties to cover the deep zones. Lee is a highly intelligent defender, boasting a good understanding of pass concepts and the ability to quickly identify routes.

When Kiffin coordinated the great Tampa Bay Buccaneers defenses in the late nineties and early noughties, he had Derrick Brooks and Hardy Nickerson at weak-side and middle linebacker.

Their athleticism and Kiffin's schemes made them stars. The Cowboys need that level of impact from Carter and Lee. Keeping them on the field will be key.

Lee missed 10 games with a toe injury in 2012. Carter meanwhile, was sidelined for the last five games needing elbow surgery. Both also had injury issues in college that affected their draft stock.'s Kevin Patra has reported that both players are healing well and close to returning to full health. They will be the driving force of the new Dallas defense in 2013.