In a flat 4-4-2 we have great wingth that can be used for effective defense and counterattacking. 1 DM is mandatory (sissoko, melo) and the could be as offensive as the dm+wingers allow. Our wingers seem rather offensive, so we cant have an AMF, but rather a CM (MARCHISIO, POULSEN)
Now where does Diego fits in all this? He doesnt, unless we compromise smth somewhere, with diego in the midfield this translated to either lack in mobility on the midfield or defensive solidarity. Both issues are somewhat adressed (better defenders+ faster wingers)
However it remains THE riddle of the season. A riddle del neri was called to solve rather soon.
If diego stays back, control the game, keeps possesion, marks a bit and provide the forwards with long vertical passes at the back of the opposing defence and some swift passes to the penetrating wingers for counterattacks. This is safe, could be effective but kind of one dimensional.
The other option is to play in the middle, but a bit deeper, so that the lack of speed such an issue and our forwards wont be isolated, or depented to crosses and forced to return for service. Thats the optimal use for Diego there, in a multidimensional front, where his inspirations (surprising vertical passes to either forward, or lobs, short sprints, or long shoots coming from him and the center of our midfield. And and the Brazilian tricky passes to either of ascending. wingers.
Form a large variety of options that cannot be faced not even by the Italian ultra-defensive systems, however, that will leave us exposed in the back and it remains to be seen if our new defense, changed at 2-3/4 can hold.
At any case, Diego will start, we cant use him as a winger and untill we get a prolific finisher, we cant use him as SS either (thats the worst case scenario, imagine us relying on the Amauri-Diego duo, to score, dear God, please DOONT! )