Carter, in his second season out of UNC, has quickly become one of the top LB's in the NFL, ably filling the void left by Lee's absence. The play of Carter (24) and Lee (26) has many Cowboys fans dreaming of a dominant 1-2 duo at the ILB positions for the next decade, but that will depend on the two being able to stay on the field.
Carter and Lee both fell into the second round of the draft due to concerns about their injury history, beginning their NFL careers on the disabled list and emerging into their own in their second seasons. That's also why DeMarco Murray slipped to the third round and he has sustained significant injuries in each of his first two years in Dallas.
I'm not a doctor, but the game of football is a brutal sport that does unimaginable things to player's bodies. The ability to stay healthy over a 16-game schedule is a skill, and so far, neither Murray, Carter nor Lee has proven they can do it.
Jerry Jones isn't exactly Bill Parcells when it comes to player evaluation; the Cowboys probably can't afford to be to picky about stuff like injury and character flags in the draft if they know a guy has talent. However, there are consequences to that approach and we are seeing them now.