Maguire, 26, became the world's most expensive defender last summer when he moved to Old Trafford from Leicester in a deal worth £80million.
But a year earlier, United backed out of talks with the Foxes when Mourinho told club bosses that paying a penny more than £50million would “embarrass him."
“But Jose's interest in signing the player came with a proviso. He said that while Maguire was an excellent player, he wasn't worth more than £50million.
“In fact, he said paying more than £50million would embarrass him.
“The club was armed with that valuation when they spoke to Leicester – who immediately made it clear they would not sell Maguire for less than £60million.
“The deal was dead before United had even made a formal bid – and when it was decided to move for Maguire again a year later, his price had gone up to £80million."