It's the confidence in those centre backs and holding player that allow you to push your full backs up as combined with the high press you force teams to kick long and if you've got 3 players mopping up all the long balls you suffocate the opposition. We did it for years with Rio, Vidic & Carrick with Rooney pressing defenders forcing them to play long it played straight into our hands.
What you need is full backs who can sprint back into position when teams do break out and recover Evra was brilliant at this he was basically a one man left side, since he dropped off in the later Fergie years we've not had a world class full back. I think what people saw with James as a potential full back was starting a bit deeper he could come on to the game and attack player with his speed as in theory he'd have more space to attack if he plays in the front 3 there's not a lot of space for him to run into.
It's hard to tell what the plan is for next season formation wise we kind of settled into a bit of a 4-4-2 diamond towards the end of last season but I'm not sure that is really the plan for Ole I guess we'll find out more in pre season but I expect to see a 4-3-3 or 4-2-3-1 I think in the 4-2-3-1 there will be more freedom for a guy like Dalot to get forward as you have that second holding midfield player.