The Dutchman started the season as one of Erik ten Hag’s assistants and stepped up as interim manager, before being let go by Ruben Amorim and swiftly taking up the job as Foxes boss.

Bobby De Cordova-Reid’s first-half header had Leicester dreaming of a fourth-round win at Old Trafford, but substitute Joshua Zirkzee levelled after United reacted to being booed off at the break.

Extra-time beckoned until Maguire headed home to make it 2-1 deep in stoppage time, sparking bedlam in the stands and fury among the Leicester ranks.

The United defender had clearly strayed offside when the ball was sent over, but VAR is not introduced until the next round.

“It’s a hard one to take because the game was decided on a mistake,” Leicester manager Van Nistelrooy said.

“It was clear. It’s not a matter of VAR, where you have to look at millimetres. It was half a metre. It was clear.

“The team didn’t deserve to lose the game in this way because they fought, they played well, dominated the game for a long bit, leading 1-0 and, yeah, that’s how you feel after this.”

Van Nistelrooy told ITV that Leicester were “not defeated in Fergie time” but “offside time” as a promising performance by his side came to nothing.

Put to the Dutchman that the dressing room will be as angry as they are disappointed after that, he said in the post-match press conference: “Yeah, we are. We’re gutted.

“When you prepare the whole week for this game and reacting after the Everton loss and put in a performance like that, and it’s been decided on an obvious, clear and obvious, mistake that is unacceptable, unthinkable in this level.

“But it happens to you, and we have nothing else to do than pick ourselves up, also from this, again. That is not what the team deserved.”

Van Nistelrooy left Old Trafford wondering what could have been, while United head coach Amorim acknowledged that his side rode their luck.

“We need a lot of things that’s nothing to do with luck,” the Portuguese said. “It has to do with quality of the work.

“We need to improve every detail of the game, our energy. My work with them has to be so much better, but sometimes you need a little bit of luck and today we had it.

“With VAR it was not a goal, and I think it’s important to have because it’s fair for the game.

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“It’s really hard to lose one game in the last minute with an offside play, but we need and sometimes we deserve a little bit of luck.”

Maguire will take the headlines but Alejandro Garnacho was man of the match, with the lively half-time introduction’s cross leading to Zirkzee levelling in a promising display.

“He doesn’t need a big change,” Amorim said of the Argentina international, who he dropped for December’s Manchester derby alongside Marcus Rashford. “We are talking about the standards.

“It’s just to train well, to receive the information, to understand we want to help him, we want to play in a different system, but it’s not complicated.

“So, he changed everything, he trains really well. He stayed in the bench five games in a row and every time he came on the pitch he was a different guy, receiving the information and playing.

“So, it’s simple things. It’s not a big thing. I am really happy. You can see it. I think he’s improving.”