Ricky, agree with your diagnosis, but not the remedy. To put Agger back in now would break up a proven partnership, and a good understanding between center backs is as important, if not important as a good understanding between strikers. Like the old saying goes, if it ain't broke, don't fix it.
Agger will get his chances, and then it will be either Carra's or Skrtel's time to prove they should be put back into the squad. He'll play in domestic cup matches, and likely European games as well, assuming we can get a decent result in Madrid. But in the prem, until either form drops, or injury hits, I'd stick to a proven formula. Hopefully risks for dropped form, or injury can be identified on the training ground, and rotation is done just when its needed, but why rotate when you don't need to?