Altough like you said we have other " creative " players, Pirlo's is the one who can control our tempo, speed up the game if he wants too and slow it down, he's the one who can link the defence with the rest of the players in a wingerless system, watch yesterday game how much we struggled to build our attack from the back, the majority of the time the defence went to our WB ( wich resulted in many miss passes ) to start the attack because Pogba won't present himself to the defenders the way Pirlo does even when he's marked.
Unless we decided to drop the regista system, i can't see how we can cope without Pirlo especially when we are playng against team that will defend with numbers.