This hurts because Mitchell is a good player and would have helped a lot since defensive coordinator Bill Young intends to rotate the linebackers to keep fresh players on the field at all three linebacker positions.
This could open up playing time for a younger player like freshman Seth Jacobs or a veteran that can play star. It is a known fact that Shaun Lewis and backup Lyndell Johnson were going to play, but the hope was to be a solid three deep at all linebackers. Mitchell's absence weakens the star linebacker depth. Mitchell is also a good contributor on special teams.
Not only does Mitchell's departure possibly hurt on the field but the Cowboys need to turn around this trend of backups leaving and the damage it does to an APR that has been slipping. Herschel Sims and players that get in trouble are one thing, you can't keep them around.
However, players that may not have reached full expectation from recruiting need to be encouraged to stay and help the program. You have 85 scholarship players but there are only 22 to 30 starting spots on offense and defense combined (based on having various personnel groups). That means some 55 or more of your scholarship players are going to be backups. You need for them to stay in your program. A good percentage leave and the APR slips into critical penalty stage.