When Messi will permanently move in the center, as he gets older, to be a trequartista (like a much better Totti), we would eventualy need a double pivot as a cover behind him. So, in that respect, it would be better if, in time, we would find ourselves a classic RW and switch to a 4-2-3-1. Someone like Verratti would be ideal for that plan, because he could form a double pivot with Busi, but he could also play a bit higher in a 4-1-4-1.
So, I don't think we need a no10. We have Messi. We need a player who is very solid defensively, runs 11-12 km when he has to, and can also pass very well.