You basically called him the best midfielder in the world by "miles".
He is definitely miles ahead of Marchisio. But if you compare him to Modric for instance Pirlo is still better in terms of passing, movement and obviously set pieces but he does less defensively and has lower stamina.
At this age he is still the best deep lying playmaker in the world.. miles ahead of most players and a little better than very few like Modric.
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Marchisio is an amazing player to have. He can play Box-to-Box, Ball Winner or Deep Lying Playmaker all to good effect. While our starting trio are ahead of him in their respective roles, none of them can play all 3 roles like that. Vidal is superior as a B2B and BW but probably not as a DLP. Pirlo is only a DLP and would suck at any other role while Pogba might be a better B2B but worse at the other two roles.
Marchisio is the perfect rotation guy in midfield.