Bewildered Manchester United boss Louis van Gaal said his side had "given the game away" after Leicester scored four goals in just over 20 second-half minutes to come from 3-1 down and beat United 5-3.
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Van Gaal looked set to oversee a second consecutive league win when United went 3-1 up after half-time in the Midlands, only for two quickfire goals to begin the turnaround.
"How is it possible to give this game away? We have given the game away," the Dutchman told Sky Sports.
"Leicester has proven already in four games that they have a strong character and that when they are 3-1 down, you can play all the game with more possession but you cannot do these kinds of things [mistakes]."
To make matters worse for United, defender Tyler Blackett was sent off after fouling Luke Vardy to conceded the penalty from which the Foxes scored their fifth goal, and Blackett's fellow defender Jonny Evans suffered an injury.
"It is not pleasant when players are injured or sent off, but we can manage," Van Gaal said. "I don't think it is the weakness of our defence, it is the weakness of our defensive organisation. We have to do that better.
"You have to know when you can pass the ball and when you have to get rid of the ball. In this game, at 3-1 I felt we had played fantastic football but then you cannot make the errors we did.
"We have to improve. We created a lot of chances, we scored fantastic goals so that is a plus -- but we lost the game, and that is the most important thing."