"Chelsea could do with some reinforcement at the back"
Same applies to Man U too, who may be one injury to Rio away from being exposed badly at the back.
The current centre-back cover of either Smalling and Evans or Smalling and Jones doesn't exactly instill fear into the oppositions's attack (imo) especially in the big games away from OT. Remember Man U have many these games in the 2nd half of the season than the other top 6 sides.
I honestly think both CB pairings would struggle without the experience and cool heads of a senior player such as Vidic or Rio beside them, hence the need for Cahill to steady the ship until the big guns recover.