As Paul Pogba makes his return to Manchester United - the club that lost him to a free transfer when he signed for Juventus in 2012 - Andrea Pirlo says he and his former team-mates could not believe the Premier League giants let him go in the first place.

Pogba moved to Juventus after failing to agree terms on a new contract with United and received compensation of under $A1.3 million, and has paid almost $130m to bring him back to Old Trafford.

But Pirlo said he is still amazed that United managed to let him slip through the cracks in the first place.

"I will always remember the first day he had training at Juventus," Pirlo said. "He was only young, but we could see he was special.

"There was almost a disbelief that Manchester United had let this player go."

But even though it has taken United four years and a lot of money to right the wrong, Pirlo says it is a plunge worth making for the English giant.

"A lot of people have a lot to say about what the fee could be and if he's worth it," he said. "But when you pay a big fee you just have to ask one question - can this player play a big part in us winning trophies?

"If he can do that then it will always be worth it for the buying club.

"We know Pogba can do this and we know he can do it for a lot of years to come."

As for any price tag pressure, Pirlo dismissed the notion that Pogba would be swayed by the amount of money United have spent on his return.

"One of the biggest misconceptions is that players care about transfer fees," Pirlo said. "For a player they do not care at all. They do not care if they move clubs for one million, 10million, or 100million Euros. That is just something a player leaves to his agent and the clubs.

"As a player, you just want to be at a club where you can achieve everything you want to."