Harry Maguire scored an injury-time winner against former club Leicester as holders Manchester United came from behind to win 2-1 and advance to the FA Cup fifth round.
It appeared the defender was in an offside position as Bruno Fernandes swung in the free-kick that he headed past Mads Hermansen, but with no VAR in action at this stage of the competition, the goal was awarded.
"We were not defeated in Fergie time, we are defeated in offside time," said Leicester manager Ruud van Nistelrooy to ITV after the final whistle.
United, who have reached the last two finals, advanced past the fourth round for the 10th time in 11 seasons.
There was hope inside Old Trafford that, against a team with eight Premier League defeats in their last nine games, United would not struggle as they had done so often on home soil recently.
It did not take long for that to evaporate, though. First the volume of the crowd quietened, then came the groans when the ball was played backwards, and the frustration at the lack of intensity and creativity. The hosts lacked drive, played at walking pace at times and looked devoid of ideas.
Leicester took full advantage of that just before the break. Bilal El Khannouss was allowed time and space to skip past Leny Yoro and pull the ball back for the unmarked Wilfred Ndidi, whose shot was saved by Andre Onana, only for the ball to ricochet off Bobby De Cordova-Reid and over the line.
Speaking on ITV at half-time, Sky Sports' Roy Keane pulled no punches with his assessment of the hosts' display.
"It's shocking. Really, really poor," he said. "No shot on target. They had two on target against Crystal Palace. They're actually boring now."
Patrick Dorgu - lively on his debut - was replaced by Alejandro Garnacho at half-time as Ruben Amorim looked to inject fresh life into his team, but it was more than an hour before they posted their first shot on target of the night.
When it did come, only an incredible last-ditch clearance from Caleb Okoli - which hit the crossbar before being hooked away - stopped Garnacho's shot from crossing the line.
United's momentum continued to rise, though, and they restored parity inside three minutes. Garnacho's cross found Rasmus Hojlund, whose connection was blocked by Wout Faes. But the ball fell perfectly for Zirkzee, who side-footed his first goal since December 19 into the empty net.
From there, it seemed as though extra-time was more likely than either side finding a winner.
But in the final seconds of second-half stoppage time, Maguire connected with Fernandes' wonderful delivery to get United out of jail.
Amorim: Good result, not a good performance
Manchester United head coach Ruben Amorim speaking on ITV:
"We have to believe it until the end, but this time 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. 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 bit more speed, winning second balls and then we managed to turn things around.
"It was a good result, not a good performance.
"I cannot focus just on saving the season or the momentum with a cup. It's not the way I see football, it's not the way I see Manchester United.
"I'm not thinking about the end of the season because the final is at the end of the season. I don't care about that, I care about the moment and the moment is not good enough.
"A good result. We move forward to the next stage. We have a week to prepare, but today was not a good day."
Van Nistelrooy: Offside decision hard to swallow
Leicester manager Ruud van Nistelrooy speaking to ITV:
"We were not defeated in Fergie time, we are defeated in offside time.
"VAR you have a couple of centimetres, a couple of inches. This was half a metre, clear in line. That's a hard one to take because the team deserved extra-time.
"It was a very good team performance. Started really well, pressing United. On the ball, very stable. Deserved to go 1-0 up at half-time.
"We knew there would be some pressure on us in the second half and chances but not even that much.
"Decisions like this, at our level, are hard to swallow."
