Every team has key pieces. The same goes for Dallas. Usually when a key piece gets injured, the team is in for a rough year. Here are the players I think Dallas needs to retain to have a playoff chance this year:
Demarcus Ware is the best player on the Dallas Cowboys. Without him, there is virtually no pass rush for the Cowboys. Victor Butler is good enough, but Ware brings a whole 'nother dimension to the defense. Without him, pressure will be tough to get on QB's and the secondary will get picked apart.
Witten accounted for the most receptions from the Cowboys last year. He is the most consistent player on the team and is vital for the pass game. He is also a great blocker that can work magic for runners. Without him, their is no security blanket for Romo, and the whole pass game will fall apart.
This should have been the first name on this list. Everything the Dallas is on offense starts with Romo. He is an elite passer, and although Orton is skilled, he does not possess what Romo has. Romo and Witten together make the offense click and the Cowboys will get sorely beaten out if he isn't under center.
Dez is a monster. He is the biggest threat on the Cowboys offense, and is a big play machine. Behind him, there are starts but not many playmaking gamebreakers to catch the ball. Every good offense needs a guy who can turn the game around and Dez is that guy for Dallas.
Carr will be the center of what will hopefully be a strong pass defense this year. He is a solid player who upgraded a porous CB group from last year. Behind him, Morris Claiborne is a rookie who cannot be expected to handle the pressure and become a leader in the secondary from day 1. If it's all up to Claiborne, the Defense will struggle. Jenkins is also a solid player, but he is injury prone and may start the season missing six weeks due to his shoulder.
Murray is another security for Romo. When the pocket breaks down and Romo struggles to keep defenders off him, he looks for Murray. Behind Murray is talented-yet-unreliable Felix Jones. Jones is injury prone and if Murray goes down, he takes with him the threat of a solid run game.