Louis van Gaal hit out at his Manchester United players after watching them crumble to an embarrassing 5-3 defeat to Leicester.
United raced into a 3-1 lead thanks to goals from Robin van Persie, Ander Herrera and Angel di Maria, who chipped Kasper Schmeichel with a delightful goal in the 16th minute before Leonardo Ulloa banged in a header.
Marcos Rojo missed a clear header from four yards and full debutant Radamel Falcao struck the bar, but Van Gaal's team then went to pieces.
Jamie Vardy and Estebian Cambiasso found the net after some terrible defending while David Nugent and Ulloa also scored penalties to seal the Foxes' first home win over the Red Devils in 29 years.
After enjoying such a comprehensive win over QPR last week, Van Gaal found the defeat at the King Power Stadium hard to take.
"We start last match against QPR very good with a new team, new players and then we played very well here, but we gave the game away," the United manager said.
"Leicester had five shots on goal, and that was it.
"These five goals were existing because we made errors in ball possession.
"We created a lot of chances and made superb goals but you have to do that over 90 minutes, not 60 minutes. It was not enough today.
"You have to kill the game and we didn't do that."
At times in the first half, United were simply irresistible.
Argentinian duo Rojo and Di Maria looked dangerous down the left flank while Falcao, Van Persie and Wayne Rooney looked a devastating front three.
But in defence, United's concentration and marking was easily exposed by the Premier League newcomers.
Eyebrows were raised when Van Gaal decided to not to spend any of his vast budget on a centre-half this summer, and it is easy to see why after this display.
Chris Smalling, who came on for the injured Jonny Evans in the first half, was easily bullied by Leicester's front three.
Tyler Blackett, one of only three players to start every game for United this year, was sent off when he hacked down Vardy to hand Leicester their second penalty while Rafael was at fault for the first spot-kick when he barged over the former Fleetwood man.
United lacked the energy to mount a comeback late on as Van Gaal was forced to withdraw Falcao and Di Maria through fatigue.
"I had to change Falcao, not because he was bad - he played very well - but because I have to build (his fitness) up. That is my problem," Van Gaal said.
"Di Maria also had a problem."
Van Gaal refused to criticise referee Mark Clattenburg for awarding the penalty against Rafael even though the Brazilian was fouled by Vardy in the build up.
"It was only 3-2 at the time, that was not a problem," he said.
"The problem was we did not keep ball possession."
Van Gaal, who is already without Phil Jones because of a hamstring tear, may now be without Evans for a while.
The Northern Ireland centre-half left the stadium on crutches after injuring his foot in a tackle with Ulloa.
"I don't know how bad it is," Van Gaal said.
"We will see tomorrow, he will probably have a scan."
Van Gaal's opposite number Nigel Pearson lauded his players after their historic win.
The Leicester boss reserved special praise for Cambiasso, who starred on his first start since moving to the Midlands club on a free transfer from Inter Milan.
Pearson said: "He had a very profound influence on the game.
"It's never easy for a player with such high expectations on him to come in against a side like Manchester United and run the show.
"Before today he had played 45 minutes of competitive football for us.
"He brings know-how, experience and quality."
Three years ago Vardy was playing his football in the Conference, but despite his amazing journey, he is in no mood to rest on his laurels.
He said: "It's massive. Obviously, it's not been an easy journey to come here, and now we are at the top, but it's all about staying here now.
"We managed to get three points today and that moves us up the table a bit."
Leicester City's supporters were chanting “we want six” well before the final whistle of this remarkable afternoon at the King Power Stadium, as Louis van Gaal was given another painful reminder of Manchester United's defensive weaknesses.