I'd reckon Nani wouldn't necessarily fit into the "diamond, but probably "1 of the 2" up top, drifting all over the place i.e. the "Welbeck" position against Newcastle or even thinking back to the dominant Milan side - the "Kaka" position (though if memory serves me correct, Kaka played more centrally, ala Kagawa or Rooney?).
Ideally I'd like to see:
-----------------RvP
-------Kagawa (tucked in a bit)------------Nani
-----------------Rooney
----------Cleverley-------Carrick
A "different" spin on the diamond if you can see what I'm getting at. I think having someone like Nani or Valencia out wide is imperative to stretch the defence and give the likes or Rooney, RvP and Kagawa space to operate in (with the added benefit of playing Nani means we can still allow all our attackers to rotate).
Also, our LB is going to need to provide the width, but with Cleverley in midfield, i think we have the legs to cover his position sufficiently.
I think we saw something close to this formation against Spurs in the 2nd half.
Against stronger teams, away from home though, I'd probably go a lot more conservative and throw on another midfielder Scholes/Anderson/Fletcher in place of either Kagawa or Nani/Valencia.
Thank god im not the gaffer!