Karl Robinson's League One team produced one of the finest performances in their history, while Louis van Gaal's men limped to a defeat that was both humiliating and utterly poignant in its symbolism. Two goals each from Benik Afobe and Will Grigg, one of them steered home superbly via his chest, sent Manchester United out of the Capital One Cup at their first attempt, a tournament in which they had reached the semifinal last season.
Though the team selected was almost entirely different from that which started Sunday at Sunderland in the Premier League, a club of Manchester United's resources should be acquitting themselves far better than this. Unfortunately, this was wholly abject.
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There could be no greater contrast between the spirit shown by the away team's travelling support, 6,000 of whom made their way to Milton Keynes, and that shown by the players. Or perhaps it is more accurate to say that the players selected by van Gaal applied themselves but played with fear.
This was not how the game began. Danny Welbeck, one of those who emerged from this match with some credit, started as if to show Manchester United exactly what they would be missing if he were sold. Yet his early surging runs in behind the defence were nothing but a false promise for the evening ahead.
Behind him, the ill-fitting midfield was overwhelmed. Nick Powell was withdrawn after an hour having failed to link with the attack; it is also a thing of head-shaking wonder that Anderson was once the recipient of the FIFA Golden Boy Award, a prize given to the best young footballer in the world.
United were out-thought and overrun throughout. Worst of all was Jonny Evans.There to provide leadership, he provided MK Dons with their opening goal, rolling the ball right to Ben Reeves deep in his own area when he could simply have cleared it. It was an appalling loss of concentration and set his team on the way to a devastating loss.
Shinji Kagawa, who began the game in attack behind the front two, may have regarded this game as an audition for a place in Van Gaal's affections -- somewhere he has fallen far from in recent weeks, given the Dutchman's comments after the Sunderland game -- but he was withdrawn with a mild concussion. It was another unfortunate event in a career that has regressed alarmingly since his glorious years at Borussia Dortmund, and he would be wise to look elsewhere as swiftly as possible. This is not a club at which to seek to rebuild so sorely ailing a career as his.
Ahead of him, Javier Hernandez looked slow in thought and deed, failing to turn home a late effort that would have made the score 4-1. Even that faint redemption was to be denied, and the Mexico striker would be equally wise to consider alternative options in this transfer window. What Angel Di Maria, who is surely passing these players in Old Trafford's revolving doors, must be thinking is anyone's guess.
Tuesday's defeat is desperately disappointing for all those footballers involved, as it will be, too, for Van Gaal. Many will question his decision to play a 3-4-1-2 formation, which he may defend as one that worked well in preseason and where Reece James -- well below his best this evening -- had flourished.
Van Gaal's tactics will be hauled under the spotlight, and they should be questioned; after all, a loss of this magnitude takes no little explaining. The harsh truth, though, is that for this short period at least, this is a club that does not know where its next win in a competitive fixture is coming from, and which will be more than a little relieved if they beat Burnley in their next Premier League fixture. It was striking, against a team so often praised for bringing through young players, that the game's best young performer was the 18-year old Dele Alli, who toyed with the away team's defence throughout.
Between MK Dons' regular punctuation of this game with goals, there were positives. James Wilson came on with about half an hour to go and showed impressive movement, almost curling one effort into the top left-hand corner of the net. Andreas Pereira combined well with Welbeck, and Ed Woodward was reminded of the investment that his owners, the Glazers, must continue to make in this club. Other than that, there was precious little comfort to be drawn. David De Gea, who had the worst place in the house of all from which to view this loss, will hopefully be able to view it as the very lowest point in the years ahead.