Manchester United interim boss Darren Fletcher insisted the "fragile" Red Devils must "dig deep" following their FA Cup exit at the hands of Brighton.

United bowed out in the third round for only the third time in the last 41 years, after suffering a 2-1 defeat at Old Trafford.

Former Red Devils striker Danny Welbeck came back to haunt his old club, after Brajan Gruda opened the scoring, while Benjamin Sesko's late header proved academic as teenager Shea Lacey was sent off soon after.

Man Utd, who were booed off at full-time, have now been eliminated from both the FA Cup and League Cup in their opening tie in the same season for the first time since the 1981-82 campaign, meaning they will finish another term trophyless.

"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).

"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 on Wednesday, 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."