Kelly did not played a single senior game at CB in his whole carreer. He is not a CB these days and can not be considered as one.
As to Agger, he has his qualities, but is massively overrated because he looks classy, but without any products of this class. And he is very poor in the whole 2013 year, or average at best, directly responsiple for almost dozen of goals during that stage.
So he should wait, until Toure, Skrtel, and Sakho are ahead of him and rightly so. He should wait.
And he will get his chance sooner, because he is Brendanīs favourite, so BR will throw him back after any slight poor performance from Sakho or Skrtel. He will keep his word about playing players that are in form, and will also satisfy his vicecaptain.