Neil Warnock believes it will take ten years for Manchester United to be a force again.
The Red Devils have not won a Premier League title since Sir Alex Ferguson retired in 2013 – and have hired four managers during that time.
Ole Gunnar Solskjaer replaced Jose Mourinho in December, but after a winning start, the Norwegian has won just two of his last 11 games.
Championship-bound Cardiff were relegated last weekend, while sixth-placed United failed to qualify for next season’s Champions League.
“It’s been a difficult period for him,” Bluebirds boss Warnock said.
“People say it might take two or three windows, I think it could be ten years, 15, who knows? They’ve got to catch up with Manchester City and Liverpool and it will not be easy.
“But they will come back and be just as good as what the other two are now.”
Warnock highlighted poor recruitment and the loss of traditional values in failing to give young players first-team chances as reasons for United’s decline.
But the 70-year-old is delighted his final Premier League game will be at Old Trafford.
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“They have wasted a lot of money,” said Warnock. “Are they any better off than when David Moyes was there? They’ve spent about £400million since then.
“Over the past few years, I’m not sure how much thought has gone into some of the signings.
“But what better place to have your final game in the Premier League than at the Theatre of Dreams?
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“No disrespect to all the other clubs – Anfield is tremendous as well – but where I come from, Manchester United was the place and it’s always got a special atmosphere.
“I think they are going to go back to the Manchester United values now – signing young players.
“That type of is what they want now. They have nothing to lose. They have got to generate that enthusiasm and verve that you get with young players.”