Harry Maguire: from up for sale two summers ago to redemption completed via a 93rd-minute nodded winner. If the defender was clearly offside when meeting Bruno Fernandes’s spiralling free-kick from the left he certainly did not care as here was his own tale of FA Cup romance.

Before the rush of Maguire’s intervention, Joshua Zirkzee’s 67th-minute equaliser appeared the lifeline United needed to keep a shaky FA Cup defence alive.

Having bested Leicester 8-2 on aggregate in two previous meetings with their visitors, United were what they are constantly under Ruben Amorim: disjointed. Introducing the jet-heeled Alejandro Garnacho at the break transformed home fortunes as he was key in Zirkzee’s strike. But to dice with being eliminated by opponents with 12 points fewer and candidates for relegation cannot happen, as was the case after Bobby De Cordova-Reid’s first-half opener.

But, cometh added time, cometh Maguire, whose intervention would have been scrubbed out if there had been a video assistant referee, and whether James Justin should have been penalised for handball for Fernandes’s free-kick is debatable too.

A porous rearguard is one of the myriad problems that plagues United so when Patson Daka out-foxed Leny Yoro and Maguire and teed up Jordan Ayew early alarm bells clanged. The No 18’s effort was fierce and straight at André Onana and the home goalkeeper saved.

A screengrab shows Harry Maguire (top) in an offside position before heading in a stoppage-time winner. Photograph: ITV Sport

Momentarily, United turned up the heat: a crossfield Diogo Dalot pass sent Patrick Dorgu sprinting upfield and United claimed a corner on the right. This was the flank Amorim fielded Dorgu on, rather than the expected left, the Dane’s debut offering a first sight of the head coach’s first recruit in action.

Amorim explained: “If you see the history of Dorgu in the last team he played a lot on the right. Sometimes you need a left foot on the right side to connect in a different way.”

The 20-year-old, signed in the winter window for £25m from Lecce, was a promising blend of pace, neat touches and potency off either boot, as with a right-footed effort aimed too high.

Yet Dorgu’s opening period offering was United’s sole glimmer of brightness. Instead, the Premier League’s third-bottom side possessed the thrust, a De Cordova-Reid burst again turning the home team.

Amorim either dropped to his haunches in despair or shook his head. Concern, too, coursed through the 40-year-old when, down Leicester’s left, Ayew flipped the ball to Luke Thomas and he aimed in a delivery Noussair Mazraoui headed for a corner. When Thomas hit this in from the right Mazraoui, again, repelled.

Crumbs was the attacking fare served up by United. A Dorgu foray claimed a corner, Amad Diallo pinged it in, Hermansen clutched the ball, dropped, those United players nearby got excited, then the keeper recovered.

Now: joy for Ruud van Nistelrooy’s men and the too-familiar sense of despair for Amorim’s. First, Bilal El Khannouss knocked Manuel Ugarte off the ball. His cross, from the left, was stabbed goalwards by Wilfred Ndidi, Onana saved, but the keeper saw the ball go in off De Cordova-Reid.

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Yet again United were sliced through as if juniors playing a seniors side. Clever from Leicester, hapless from the opponent who conceded on 41 minutes and, when Michael Salisbury blew for the break, the whistle was followed by the sound of boos from a supremely disgruntled Old Trafford congregation.

Manchester United appeal for offside as Bobby De Cordova-Reid (second right) celebrates his opener. Photograph: Richard Sellers/Allstar Picture Library Ltd/Sportsphoto

For the second half Dorgu was replaced by Garnacho in a reshuffle that had Diallo taking the Dane’s berth. A strange decision from Amorim to remove his team’s sole flicker of hope? Certainly, unless this was about the player having taken a knock or having only 45 minutes in him as he moves up to speed with his colleagues .

Garnacho, instantly, burned down the left and splayed Leicester: a fine start but he failed to find a teammate and once more a United attack petered out. Leicester had more fizz. Ugarte, enduring a poor display, could not stop the live-wire El Khannouss swerving around him and on the No 11 went, United just about stopping Daka prospering from the winger’s run.

With the clock running down on the holders’ chances Amorim removed Kobbie Mainoo for Zirkzee and within moments Ugarte sent in Garnacho. At close range he unloaded, the ball ricocheted off a Wout Faes’s boot over Hermansen but Caleb Okoli smashed the ball against the bar and the keeper grabbed it.

Soon, Garnacho’s zest created the leveller via a shoulder-drop and killer ball in: Rasmus Højlund’s shot was blocked and Zirkzee finished. Towards the end, Garnacho had a golden chance to set up either of the forwards but failed.

Now, though, up stepped Maguire. He said: “A great finish to the game. The second half we played a lot better. The first half was nowhere near good enough. We probably deserved to go into half-time losing.”