Harry Maguire headed a last-gasp winner as FA Cup holders Manchester United came from behind to beat Leicester 2-1 on Friday, denying Ruud van Nistelrooy a fairytale return to Old Trafford.

Bobby De Cordova-Reid put the Premier League strugglers ahead late in the first half of the fourth-round tie against a pallid United, who have lost their fear factor at home.

But the introduction of Alejandro Garnacho at half-time 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 former Leicester defender Maguire headed in a Bruno Fernandes' free-kick deep into stoppage time.

The Portuguese manager, who is struggling to implement his ideas at Old Trafford three months into his reign, said it was a "poor performance" from his side.

"We didn't have any energy in the beginning, especially in the first half," he told ITV. "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 but 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."

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.

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. (AFP)