Will Grigg and Benik Afobe scored two goals apiece as a young Red Devils side were humbled in the League Cup second round to leave the Dutchman still without a victory
Van Gaal picked a mixture of inexperienced young players and those on the fringes of his squad, including Danny Welbeck, Javier Hernandez, Shinji Kagawa and Anderson.
But a Jonny Evans error allowed Will Grigg to open the scoring in the first half, sweeping the ball past David de Gea from close range.
With United chasing an equaliser after the interval, Grigg made it 2-0 with a clever chested finish from an excellent Ben Reeves cross and it was Arsenal loanee Benik Afobe who came off the bench to secure the victory with two goals to mark an historic night for the home side.
|By Dan Brett
It was a night where many would have expected the Premier League side to run riot, especially given their poor start to their league campaign. However, they were greeted at Stadium:MK by a plucky League One side with nothing to lose.
Louis van Gaal's men started brightly and looked most likely to score, with strike duo Danny Welbeck and Javier Hernandez causing all sorts of trouble behind the Dons' back four, but Jonny Evans's mistake opened the floodgates for a cup upset.
Will Grigg bagged two as Benik Afobe netted his own double from the bench on an inspiring night for the Football League side. The perfect performance for Karl Robinson's men but not even Angel Di Maria's arrival will lift Red Devils' spirits after this thumping. United never got out of first gear and top stars Welbeck, Hernandez, Shinji Kagawa, Evans and Anderson failed to deliver on the night. Not what you would want to show the man who is tasked with your future at Old Trafford.
A United team loaded with players determined to catch Van Gaal's eye started confidently as Danny Welbeck, in particular, caused problems. The visitors' endeavour almost brought immediate reward, Nick Powell curling just over the crossbar.
MK Dons came back into the game, although their possession was not producing opportunities. However, Karl Robinson's men were gifted an opener in the 25th minute by an Evans howler. The centre-back's pass from defence was intercepted by Reeves, whose simple ball left Grigg with the easiest of taps-in.
That seemed to knock the wind out of United's sails, although Powell threatened to reinvigorate his side when firing just wide from range with the outside of his foot.
Things almost went from bad to worse after the interval, when MK Dons were denied a penalty when Evans appeared to have handled in the area, but United could not take advantage of that break, Welbeck curling a powerful effort just wide of the post.
And their misery was compounded in the 63rd minute, Reeves and Grigg combining again in a fine move that ended with the latter cleverly chesting home his team-mate's perfect left-wing cross.
Any thoughts of a United revival were snuffed out by Grigg's replacement, Afobe, who then slid home easily after being picked out by Danny Green.
James Wilson almost levelled, United's first shot on target coming in the 72nd minute when David Martin produced a superb one-handed save to deny the highly rated striker's 20-yard strike.
Adnan Januzaj rattled the post but, if anything, Van Gaal's side were lucky not to lose by more with David de Gea forced to turn away Dean Lewington's cross.
Relief at that quickly turned to further despair, however, as Afobe bundled his way into the box and volleyed home to create a scarcely believable scoreline.
Hope continued to come in the shape of Wilson, whose header again forced Martin into an excellent save, although that was scant consolation for Van Gaal.