Joao Felix capitalised on slack marking from the defender to power home a superb header just seven minutes into the clash, handing the visitors a mountain to climb.
The £80million centre back Maguire has been slated on a number of occasions by furious Red Devils supporters this season, and they are far from happy once again.
Then, the diminutive Felix – who stands at just 5ft 9in – escaped the attentions of both Maguire and Raphael Varane to head the ball against the post and in.
It was a moment of class from the attacker, who cost Atletico a whopping £113million, but Maguire’s slow movement and inability to react have been targeted.
One Twitter user said: ‘Manchester United have one of the worst CBs in the game right now. Lol. Maguire is garbage!’
‘Maguire’s cost us again,’ another wrote. ‘Would love to see our goal differential when he’s on the pitch.’
A third posted: ‘You can’t hide anymore, Maguire. Is [sic] time to go.’
‘Maguire gotta be the most expensive fraud in footballing history,’ another said.
A fifth, meanwhile, added: ‘Maguire moves in slow motion.’