Cause for concern over the Cowboys' new defense?
First, we have the linebackers. The Cowboys just signed Justin Durant, formerly of the Lions, to round out their three starting linebackers. After racking up 103 tackles in 2012, Durant is penciled in on the strong side. He's not a great player, but ranked a solid 18th among 4-3 outside linebackers last season, according to Pro Football Focus.
On the weak side the Cowboys will have the promising Bruce Carter, who was having an outstanding season before an elbow injury sent him to the injured reserve in 2012. He had 70 tackles in 11 games before his injury last year, which means he was on pace for 100+ tackles. His speed makes him a great fit in the 4-3 and has people comparing him to Derrick Brooks.
Now we have the man in the middle, Sean Lee. A Pro Bowl caliber player, Lee was limited to six games due to a toe injury in 2012, but his impact was felt in the games he was able to play. In the five games that Lee started and finished, the Cowboys held their opponents under a hundred rushing yards four times. In the ten games that Lee didn't play, the Cowboys gave up 100+ rushing yards eight times. Lee was arguably the league's best inside linebacker before his injury, and his presence made a big difference against the run.
If the linebackers can play to their full potential, they can partially offset the lack of size on the defensive line. The linemen, however, have to make sure they are able to eat up blockers and give the linebackers some room to make plays.
The other players that needs to step up are the cornerbacks. The Cowboys gave Brandon Carr a hefty contract to come to Dallas last season, and traded a first- and second-round pick to move up and draft Morris Claiborne. Carr had a strong debut season in Dallas, and Claiborne, though he tailed off towards the end of the season, showed flashes of why he was worth a 6th overall draft pick. Claiborne is also expected to focus on getting physically stronger this offseason, which should help him improve his play.
Although some people suggest that Kiffin's Tampa 2 scheme makes the Cowboys' corners less significant, I don't believe that is the case. Although they focus largely on covering the flat, their press coverage will be crucial to the pass defense - both Carr and Claiborne play great press coverage. On passing downs, the combination of Ware and Spencer rushing the passer while Carr and Claiborne are jamming the receivers could force quarterbacks to get rid of the ball before long routes have a chance to develop. As a result, the Cowboys could mask some of the weaknesses on the back end of their defense.
I'd still like to see the Cowboys address the safety position (with a free agent like Michael Huff or a player in the draft), but if the cornerbacks can give receivers some serious problems getting off the line of scrimmage, Ware and Spencer should make it hard for opponents to take advantage of the defense's biggest weakness.
The Cowboys could really be a boom or bust defense next season. They definitely have some significant weaknesses, but if the linebackers and cornerbacks can be exceptional (which they have to potential to be), they could field a strong unit on the defensive side of the ball.