If we have an injury crisis and lose Carra, Agger our options would be Kelly, Skrtel. Is that enough or do we need a another center back? Talk of Cahill coming in but I don't see him displacing Carra, Agger so would he be happy on the bench?
He doesn't have to "displace" them. Rotation isn't just a Rafa thing- all managers need to do it somewhat to keep players fresh. If when all fit Carra, Agger, and Cahill each play 2 games out of 3 I can't see them complaining too loudly- they may appreciate a Saturday off so they can mow the lawn.