Just seven days after recording his first win as Manchester United manager, Van Gaal was brought down to earth by the Barclays Premier League newcomers.
Lax defending allowed the Foxes to come back from 3-1 to complete their first home win over the 20-time English champions in 29 years.
Van Gaal, who spent over £150million in the transfer window, is refusing to panic despite the fact that his team have taken just five points from their opening five matches.
"I cannot say that I am happy, but in the world of football, these matches happen," said Van Gaal, who ended up with 10 men in his side following Tyler Blackett's red card.
"In my first year as trainer-coach of Barcelona we were 3-0 ahead (against Valencia) with a quarter of the match to play and we lost 3-4.
"It can happen.
"We had the game in our pocket, but we gave it away
That I don't like, but I am convinced we shall improve."
United's attacking talents were clear for all to see in the first hour, when goals from Robin van Persie, Ander Herrera and Angel Di Maria, who beat Kasper Schmeichel with an outrageous chip, found the net.
But, buoyed by the defensive lapse that allowed Leonardo Ulloa to head in after Di Maria's wonder goal, Leicester continued to attack and the pressure paid off in the second half.
Rafael and Blackett fouled Jamie Vardy to hand the hosts two penalties, which were dispatched by David Nugent and Ulloa respectively.
Vardy, who was plying his trade in the seventh tier of English football four years ago, also found the net in a frantic final half-hour, as did Esteban Cambiasso, who was making his full debut.
Former captain Gary Neville criticised his former side, branding them as "soft-centred", and Wayne Rooney hit out at his team's defending too.
"Overall, it wasn't good enough as a team," the United captain told MUTV.
"We played some good stuff at times and scored some great goals, but we made too many mistakes as a team and have to do better."
Van Gaal, meanwhile, lashed out at his players for failing to kill the game off.
"We made such big errors," said the 63-year-old, who handed Radamel Falcao his debut at the King Power Stadium.
"We did not keep possession."
Van Gaal, who is already missing the injured Phil Jones, could now be without Jonny Evans for a while after he hobbled off following a challenge from Ulloa.
"I don't know how bad it is,'' Van Gaal said.
''We will see (on Monday), 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 joined on a free transfer from Inter Milan this summer.
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."
Source: PA