Ideally you need both. Sneijder | another deeper M | Carrick/Fletch.
That 'missing' piece #2 in midfield should/could be anderson. Perhaps. This coming season is "make or break" from the lad. Otherwise, we'll have to go to the market for two midfielders.
Sneijder isn't as '2nd striker' ish as van der vaart is for example.
As for wazza and the #10, why can't be play false 9 a la messi? He's done it before as he's dropped deep. Forget europe, but in the league when we played ronnie, tevez, and wayne together, wayne would drop false 9. Does having iniesta stop messi from being at the 'tip' of the midfield tons of times?
If you want to shoehorn hernandez as the cetnral striker, then wayne will have to play wide left. I agree, he struggles with this and can't be as flexible as someone like eto'o though wayne is just as naturally gifted if not moreso.
Ideally I'd like to see this:
Nani------Wayne-----Sanchez or Wayne----Hernandez---Nani/Sanchez
with a midfield of:
Sneijder, Ando/new signing, Carrick
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Mind you, this doesn't have to be done ALL at once this off season. However over the course of two summer windows, I'd like the team to be able to go to this lineup for europe if fergie wants to win it back within 3 years.