I have an inkling that Rio is going to leave at the end of the season. That would leave us with Vidic, Evans, Smalling and Jones. We'll know by the end of the season if Vidic can be available for majority games or it will be start/stop for him from now on.
Anyways, I think it is ideal to have 5 CB's with the number of game we play -
Two experienced - Vidic and Evans
Two up and coming - Smalling and Jones
One really young CB - ??
Plus, Jones, Smalling and Evans have their injury troubles from time to time. So, I don't think it will be a bad idea to get another player in the mix.