The England international was made the most expensive defender in world football when United paid Leicester a cool £80m for his signature.
But former Spurs and Real Madrid playmaker van der Vaart says the 26-year-old isn't worth the money.
"When I go to see an amateur club on Sunday, I can find three players who can play like him (Maguire). I mean that," claimed Van der Vaart
The Dutchman continued: "It might be silly to say it, but he won't hear it anyway.
"We are talking about a €90 million (£80 million) signing here. (If Maguire is worth €90 million) then Van Dijk is worth €300 million (£258 million)."