David de Gea's struggles at Manchester United have gone beyond the point where they can be considered a blip, according to the club's former captain Gary Neville.
The Spain international made a costly mistake against Tottenham on Friday as he failed to keep out a shot from Steven Bergwijn.
Ole Gunnar Solskjaer's side went on to salvage a 1-1 away draw thanks to Bruno Fernandes' late penalty.
But De Gea's error prompted an astonishing rant from another ex-United skipper, Roy Keane, who branded him "overrated" and said he would have been tempted to punch the Spaniard in the dressing room.
Neville has similar concerns and feels United can no longer rely on the 29-year-old, whose problems date back to the World Cup.
"De Gea was the one player United could rely upon for the last four years but he is not the same," he said on the Gary Neville Podcast.
"He hasn't been the same for some time. When form drops for six months it's a blip but when it goes on for a year you start to worry. When it goes over two years it becomes more permanent.
"That is now a fair representation of what De Gea is as he's making lots of mistakes. Ones he would never make – he was always somebody you could completely rely upon.
"The reception he's got in Spain has affected him. He's even been booed by Spanish fans when he's wearing the Spanish shirt.
"In the World Cup in 2018 he really struggled and from that he's doubted himself."
Neville believes the problems for De Gea – who joined United in 2011 – are in the mind, urging him to get back to basics and work harder than he has done at any other point of his career.
"It can only be a confidence thing," said Neville. "Mentally he's not quite the same. The arms, legs and body are the same – it's got to be the mind.
"He's got to go back to basics. There's only one thing you can do when you're having a tough time: work, work harder than you have ever done in your life.
"What will happen is that you will tell yourself the amount of work you put in you deserve to be good again. That means coming in at 7.30am and leaving at 6pm if needs be.
"He has to do something to stop the erosion. Mistakes like that can happen on the first game back but it's happening far too much over the past couple of years."