In front of a decent size crowd at St Andrew's, Keith Bertchins side slipped further away in the title race, going down 3-0 to Ricky Sbragia's free scoring Red Devils side whose win see's them leapfrog Blues in the Barclays Premiership Reserves League table.
What made the result even more surprising was that the Blues boss fielded such experienced players as Darren Anderton, Robbie Blake, Darren Carter and Tiny Taylor whilst his United counterpart sent out a team made up mostly of youth team players!
David Poole opened the scoring for the visitors on 35 minutes and five minutes before the break United doubled their advantage when Colin Heath capitilised on a poor piece of goalkeeping by Nico Vaesen. The Blues keeper failing to clear what looked a simple back pass from a defender.
United striker Floribert Ngalula wrapped up the points for the visitors on 61 minutes with a well hit 25-yard free-kick that Vaesen may well once again have done better with.
Sam Alsop, Njazi Kuqi and Andrew Barrowman all went close for Blues but in the end they were well beaten by the young United side.
The result saw Blues slip to fourth in the league table, four points behind local rivals Aston Villa at the top and having played two games more.
Blues: Vaesen, Parratt, Alsop, Oji, Martin Taylor, Carter, Robbie Blake, Anderton, Kuqi, Barrowman, Motteram. Subs: Legzdins, Birley, Till, Painter, Hall.
Man Utd: Heaton, Picken, Eckersley, Pique, Spector, Fox, Heath, Ngalula, Poole, Rossi, Martin.