Arsenal 3-0 Manchester United: Louis van Gaal in shock after slumping to humiliating defeat
Louis van Gaal was "amazed" by Manchester United's display as Arsenal got back to winning ways in some style.
An air of positivity had been building around Old Trafford after a run of four straight wins in all competitions.
United began the weekend top of the Premier League standings, but have now been usurped by Manchester City and Arsenal after an embarrassing 3-0 defeat at the Emirates Stadium.
Exceptional Alexis Sanchez efforts either side of a fine Mesut Ozil strike meant the match was over inside 19 minutes, leaving Van Gaal as perplexed as he was angry.
"I didn't expect that," he said. "I was surprised - not performing our game plan, not the will to win. I didn't see that.
"We were top of the league so I was surprised - amazed, maybe that is a better word.
"When you give a team like Arsenal so much space, then you know that you shall lose.
"And we have prepared ourselves in that way, to play more contact but don't lose your aggression. It was amazing for me and I am very disappointed."
Van Gaal bemoaned United's lack of intensity, aggression and pressing after defeat in north London - the worst possible way for his side to approach the international break.
"That I don't know," the Dutchman said when asked why the encounter had played out like that. "I put the question already at half-time and also after the match because normally I say the things how I see it.
"But, yeah, that is also difficult to answer for the players because they are all in an emotional moment.
"We have lost, we have lost in a way that you cannot lose when you are top of the league - you cannot start like we have started and they know that.
"The worst thing is that everybody is going away.
"They are flying away and I cannot build up confidence to play football, to win your next battle against Everton away, for example."
Van Gaal began to clam up after giving those initial answers in the post-match press conference, during which he defended the decision to leave Morgan Schneiderlin on the bench.
There were no selection questions for his Arsenal counterpart, though, which must have been a relief for Arsene Wenger after a tough few days dominated by the decision to play David Ospina.
The Frenchman bristled this week when asked about the choice to leave Petr Cech out of the starting line-up in Tuesday's shock 3-2 defeat to Olympiacos - a result Arsenal bounced back from impressively.
"I've been at the club for 19 years and I know what you want is a strong response in a big game when you have a big disappointment," Wenger said.
"But we (took it in) perspective and focused on what is important. It's us and not too much what was said and we focused on what is important in the game and that is part of being at the top level.
"The pressure on any disappointment today is massive and it's maybe a little bit more difficult to deal with it but we did well."
Ozil, Theo Walcott and Sanchez were amongst the standout performers for Arsenal, with Wenger downplaying the latter's injury as a "little groin problem".
However, there was certainly no talking down the importance of a result that puts them two points off leaders City.
"To win in a big game is always a statement," Wenger said.
"We are in it, we are two points off the league leaders, Manchester City, so I hope that result today will give us belief to fight for it."