Johnson can be the one to provide width and remain in the opposition half, Agger only has to remain solid and give the likes of Coutinho, Suarez and Sturridge to attack with impunity. I liken the dynamic to Carra and Babbel.
thing is Coutinho starts as a LM and drifts everywhere.
so you really need an attacking/energetic fullback to keep using that left wing, running up and down non-stop otherwise it just becomes acres of space for the likes of coleman and mirallas to do with that they please.
Sakho is not a LB, and i dont think he should play there, but out of him and agger it should be him.
personally i would just play agger and another CB, all 4 are class to be honest, i just like the calmness agger brings to the back four.