I thought it wasn't possible to sell staff if you did not have another staff of the same position to take their place.
That is if you only had 1 CD, the game would prevent you from selling them. Only if you had 2 CDs would the game allow you to sell one of them.
Also, if your CD's contract were to run out and you did not have another CD in your reserves, the game would automatically generate a 0 star CD for your team.