Manchester United boss Ruben Amorim said his side's last-gasp winner against Leicester City in the FA Cup fourth-round showed the team believed until the final whistle, but it had "nothing to do with Fergie time."

In one sense, Friday night's result brought a familiar feeling from the days of Sir Alex Ferguson, when late winners became as synonymous as the titles his sides won.

However, it was the sluggish first-half performance that drew Amorim's ire after the match.

"We had to believe until the end but this game has nothing to do with the time of Fergie. I think the performance, we have to do so much better with the ball, without the ball," Amorim told ITV after the match.

"We didn't have any energy in the beginning, especially in the first-half. Then in the second half we played a little bit better, with a little more speed, winning second balls. Then we managed to turn things around so it was a good result not a good performance.

"The coach is the first responsible, when one team doesn't perform, doesn't improve, it is the coach but we are here to do things and to see the game, to study the game and try to improve for the next game."

United failed to manage a shot on target in the first half at Old Trafford -- the third time this season they have done so, according to ESPN Global Research.

United went behind before half-time when Wilfred Ndidi's shot was blocked by goalkeeper André Onana, only for Bobby De Cordova-Reid to head in the rebound from close range.

A strike from Joshua Zirkzee levelled the scores in the second half, and the game appeared to be heading into extra time before Maguire headed in the winner in the 93rd minute from Bruno Fernandes' free kick, seconds before the final whistle.

"I think it's the small things, if you see the games sometimes it's to control the ball, to have the ball, don't give the ball away in the first pressure. It's the small things, it's the big things, it's everything," Amorim added.

"It's this moment, it's hard away but especially at home but we have to cope with that and we will try to do it next game."

Leicester boss Ruud van Nistelrooy let his own displeasures known after the game, pointing to Maguire being in an offside position when the free-kick was delivered.

"This (loss) wasn't necessary," an unhappy Van Nistelrooy told ITV. "This was half a meter [offside]. Then extra time, stay in the game, you never know what happens.

"We deserved to go into extra time and maybe penalties. Decisions like these in our level are hard to swallow."

VAR will be used in the FA Cup for this first time this season in the fifth round.