Pablo Barrios and Ademola Lookman notched the goals as Leny Yoro was sent off for the underwhelming visitors in the Spanish capital.
Michael Carrick’s side fell a goal down inside 53 seconds as Barrios scrambled home via a deflection off Ayden Heaven after excellent play down the left by Alex Baena.
Bryan Mbeumo and Bruno Fernandes spurned big chances for United, who were missing some key names through injury ahead of their trip to the Estadio Metropolitano.
They were fortunate not to be two down at the interval as Johnny Cardoso’s arrowed finish was chalked off for a handball by Marcos Llorente in the build-up, but substitute Lookman gobbled up the rebound from Jonathan David, who had forced Senne Lammens into a spilling save.
That wasn’t before Yoro had been given his marching orders in dubious circumstances, with the VAR adjudging the Frenchman to have hauled David down illegally and as the last man to deny a clear goalscoring opportunity.
It leaves United now winless in five games in all competitions, as the pressure heaps on Carrick.
More to follow.