Van der Vaart's scathing summary came after United shipped a late goal in their 1-1 draw with Premier League leaders Liverpool at Old Trafford on Sunday.
Ole Gunnar Solskjaer's side were on course for just their third clean sheet in nine games before Adam Lallana's late equaliser rescued a point for Jurgen Klopp's men.
But despite David De Gea's dramatic recovery from a groin strain - and £130m of summer recruits in the United defence - they were unable to keep Liverpool out.
Around £80m of that was spent on England international Maguire, and Van der Vaart, who spent two seasons in the Premier League with Spurs, feels United grossly overpaid for the former Leicester man.
"When I watch an amateur game on Sunday, I find three players playing like him [Maguire]. I'm serious," Van der Vaart said on Ziggo Sport.
"It may sound silly, but he will not hear it anyway. After all, we're talking about a €90 million transfer here. Then [Virgil] van Dijk is worth €300 million."
“It’s funny because during the Nations League, I was doing work for Dutch television,” he added.
“And I remember we were saying ‘Harry Maguire was by far the worst player on the pitch’. And then, two months later, €90m."