in regards to Melo, I believe that you are speaking of Vidal as his immediate replacement.
With Pirlo all but guaranteed a starting spot, a midfielder with the characteristics of Vidal was needed.
Someone with a tremendous amount of pace, able to play box to box, and more importantly, the stamina to be able to do it week in and week out.
There were more than an handful of matches that I had seen last year where Melo, even with him being required to cover less space than what Vidal is being asked to cover, look absolutely out of breath by the 70th minute. This year, the stamina to be able to do cover for Pirlo is a greater task than Melo covering for Aquilani last year.