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Brighton was forced to navigate a nervy ending at Old Trafford after Benjamin Sesko halved the deficit late on, but the visitors held on to progress after Shea Lacey was sent off.

Both sides made a lightning start to the encounter, with Jason Steele rushing out of his goal to deny Diogo Dalot's attempt inside the opening two minutes.

Dalot then blazed high over the crossbar, before Bruno Fernandes forced another smart stop from Steele soon after thanks to a quick free-kick routine with Kobbie Mainoo.

But for all of their pressure, United fell behind when Brajan Gruda thumped the ball home in the 12th minute, after Georginio Rutter saw his initial header cleared off the line.

Senne Lammens then almost gifted Brighton a second two minutes later after his pass was cut out by Welbeck, but the Belgian recovered well to deny the Brighton forward.

United continued to turn the screw before the break as Lisandro Martinez, Fernandes and Matheus Cunha came close, but Brighton doubled its lead just after the hour.

Joel Veltman's pass down the left was gathered by Gruda, who laid the ball off to Welbeck, with the latter taking a touch before rifling a fierce left-footed drive into the roof of Lammens's net.

United got one back five minutes from time when Sesko nodded Fernandes's corner home, but hopes of a comeback were dashed when Lacey picked up two yellow cards in the space of two minutes.

Manchester United interim boss Darren Fletcher insisted his fragile side must now dig deep after it was booed off at full-time.

"Out of cups, only Premier League games to play for, the fans weren't happy at the end, but they have got every right to air their grievances," Fletcher said. "It wasn't toxic, but they showed their disappointment.

"I'm sure if the players show the right reaction, the fans will get behind them like they always do, because there is still a lot to play for in the Premier League this season.

"We started OK, and then the goal knocked the stuffing out of us. I challenged them at half-time to move the ball with tempo instead of just composure.

"When we got back to 2-1, I thought: 'here we go', the crowd was up, the energy was up, but we couldn't get it over the line and get an equalising goal.

"I think you could see the players are fragile, but they've got to respond. Confidence is one of the most powerful things in football, so when you don't have it, you have to dig deep and then the confidence comes back.

"It's over to them, they need to make sure that they have got a lot to play for this season. This team is still good enough to achieve success this season, but they have to dig deep."

Welbeck, who left Old Trafford for Arsenal in 2014, has now scored eight goals against United in all competitions, his joint-most against an opponent in his club career, also eight against West Ham United.

"To come here in the FA Cup – whether it's the FA Cup or Premier League, it's always a massive occasion – it's one that we're delighted with, to get into the next round," he said.

"There are a few boys that haven't been playing regularly, so to put on the performance that they did was huge. It shows the depth of the squad, everybody's working towards one goal, and we're all buzzing inside the dressing room."

It was a second positive result for Brighton in Manchester in the space of five days, with the Seagulls also holding Manchester City to a 1-1 draw in the Premier League last week, and Fabian Hurzeler believes his players' performances have highlighted their "ambition".

"I'm very happy for the fans because they always give us great support," he added. "They had to travel to Manchester twice, and it's not the closest place for us, so I'm really happy for them.

"We always have the belief to compete with the best teams. You need to have this if you want to achieve something. We are a club with ambition."