Manchester United came from behind to beat Leicester 2-1 on Friday, keeping their FA Cup defence alive and denying Ruud van Nistelrooy a fairytale return to Old Trafford.
Bobby De Cordova-Reid put Premier League strugglers Leicester ahead in the fourth-round tie in the final minutes of a first half in which the home side looked bereft of attacking ideas.
But the introduction of Alejandro Garnacho at halftime gave United a different dimension and fellow substitute Joshua Zirkzee levelled midway through the second period.
Ruben Amorim's team pushed for a winner and it finally came when Harry Maguire headed in a Bruno Fernandes free-kick deep into stoppage time.
United scored eight goals in two games against Leicester earlier in the campaign during Van Nistelrooy's brief spell as interim boss at Old Trafford following the sacking of Erik ten Hag.
But this time their former star striker was in the opposition dugout, taking a break from the rigours of the Premier League, in which Leicester have lost eight out of their past nine games.
United, who beat Manchester City in the final at Wembley last May, were short of confidence after five defeats in their past seven matches at home in all competitions.
The home side were tepid in attack in the early stages and Leicester's Jordan Ayew tested Andre Onana.
Defender Patrick Dorgu, making his debut after his move from Lecce late in the January transfer window, fired over and there was a heart-in-mouth moment for Leicester goalkeeper Mads Hermansen, who nearly fumbled the ball into his own net.
But 2021 FA Cup winners Leicester gave the Old Trafford crowd a familiar sinking feeling in the 42nd minute.
Manuel Ugarte gave the ball away near the left touchline and Bilal El Khannouss got to the byline before pulling the ball back to Wilfred Ndidi.
Onana saved Ndidi's shot with his feet, but De Cordova-Reid nodded home from close range.
United's players traipsed off at halftime to a chorus of boos after failing to manage a shot on target.
Amorim brought on Argentina international Garnacho at halftime for Dorgu in a desperate search for inspiration.
Rasmus Hojlund burst into the box in the opening minutes, raising the temperature among the fans.
Garnacho looked as though he had levelled after bursting into the box but his deflected shot, which looped over Hermansen, was acrobatically cleared off the line by Caleb Okoli.
Moments later United were level after more good work down the left by Garnacho, who was terrorising the Leicester defence.
Hojlund's flick from Garnacho's cross was blocked. but Zirkzee tapped into an empty net for just his fifth goal of the season.
United looked the more likely to find a winner but it did not arrive until the 93rd minute, when Maguire headed home.
The England defender appeared to be in an offside position when the ball was played but there was no VAR.
United, 13th in the Premier League, ended the January transfer window lighter than when they started in terms of attacking options after allowing Marcus Rashford and Antony to leave.
Argentine defender Lisandro 'Licha' Martinez was missing after sustaining cruciate ligament damage during last week's defeat against Crystal Palace.
United's players wore training tops with the message "Mucha Fuerza Licha" (Great Strength Licha).