David Beckham has urged Manchester United's flops to prove they are motivated to play for the troubled club.

Former United star Beckham was in Manchester on Thursday to attend the premiere of a new documentary about the club's 1999 treble-winning side.

Beckham was a key figure in the United team that won the Premier League, Champions League and FA Cup under legendary boss Alex Ferguson.

Now the former England captain's Studio 99 company have played a role in the production of '99', a three-part series that launches on May 17.

Twenty-five years on from his role in that epic treble triumph, Beckham addressed the current malaise at Old Trafford.

United manager Erik ten Hag is fighting to avoid the sack with his team languishing in eighth place in the Premier League after Monday's embarrassing 4-0 loss at Crystal Palace.

Beckham believes the players must take their share of the blame, saying: "You'd hope that the manager doesn't have to do much.

"I think that's the whole point of being a Manchester United player and playing in these big games - you should be motivated."

Ten Hag's difficult second season in charge could be salvaged if United beat arch rivals Manchester City in the FA Cup final.

And Beckham hopes United's current crop can prove they deserve long-term futures at the club by shining at Wembley as they look to avenge last season's loss to City in the final.

"When you're a professional footballer and you're doing the job you love and you're at the club you love, when you're playing in any game, whether it's a regular season game or whether it's an FA Cup final against your rivals, that should be enough motivation," he said.

"And, if it's not, then you're in the wrong team and you're playing the wrong sport. We went into every game, whether it was a youth cup game, a friendly game, off-season or a European Cup final, with the same mentality and the same drive.

"We all know Erik's a very qualified manager and a good manager and he has the right motives."

Beckham, a member of United's famed 'Class of 92' that also included Paul Scholes, Gary Neville and Nicky Butt, won six Premier League titles, one Champions League and numerous other honours during 11 years before joining Real Madrid.

The 49-year-old, a boyhood United fan, admits he has been discouraged by the club's problems since their last Premier League crown in Ferguson's final season in 2013.

"Obviously it's been a tough five, 10, 15 years at United and we all know that but it was never going to be easy when the boss left, (Roy) Keane left, Giggsy (Ryan Giggs) left, Gary (Neville) left," he said.

"All of these important players and personalities and characters all left United, it was always going to be a difficult time.

"We want the lads to feel - from this documentary - the inspiration of being this successful."