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 atop 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 to play football, then you know that you shall lose.

"And we have prepared ourselves also 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 in the game.

"But, yeah, that is also difficult to answer for the players at this moment 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 bill or top of the league - you cannot start like we have started and they know that.

"The worst thing is that everybody is going away, even out of London airports.

"They are flying away and I cannot build up a new situation that they have the confidence to play football, to win your next battle against Everton away, for example."

Arsene Wenger was happy to see Arsenal recover their focus after being rocked by a 3-2 home defeat to Olympiacos five days ago.

And the French manager, who reserved special praise for forward Theo Walcott, hopes the Gunners can take their steely approach to domestic fixtures into European ties, with a double-header against Bayern Munich looming.

"We lost on Tuesday night, the performance was not disciplined enough. We acknowledged that and responded today," he said.

"It's about focus. Subconsciously we think we can just turn up in the Champions League... while in the Premier League we are on our toes and know it will be difficult.

"Maybe that's where we have to make a step up, but it will not be the same against Bayern of course!"

On Walcott, he added: "It was the most committed I've seen him to the cause for a long time.

"He took time to recover from that bad injury but his performance today was outstanding."

A fifth win of the season boosts Wenger's men to within touching distance of table-toppers Manchester City, who thrashed Newcastle 6-1 on Saturday.

"We are second in the league and two points off the lead," he added. "We have played some great games and won four away games, at places where it's difficult to win, like Leicester and Crystal Palace.

"I have never had a team that plays from the first game to the 50th at exactly the same level. Unfortunately it doesn't happen but I would love that."