I think Tim is right.
Pirlo would theoretically be for Wilshere what Bergkamp was for Henry. In my opinion he's been the best playmaker in the world the last decade, and easily the one with the widest repertoire. Not even Xavi has been close. The way he plays makes age much less of a factor to consider too. The main problem is that we'd end up with a very unbalanced midfield. In Milan Gattuso is positioned a bit higher up to apply more pressure on the opponents right away, thus allowing Pirlo to be more effective from deep. If we lose Song for him that task would be entirely down to Jack instead. He'd have to play instead of Jack rather than alongside him, and he doesn't press well enough to do that.
I'd be very, very worried if he ended up at either United or Chelsea though, because both of them are set up brilliantly to accomodate someone like him.
