'David de Gea would be the best signing' - Phil Neville
Neville says keeping Spaniard 'the biggest thing' for Van Gaal
Forget Gareth Bale or Mats Hummels, Manchester United's most important signing this summer will be David de Gea, according to former Red Devils defender Phil Neville.
The Spaniard has wowed fans with his displays this season and has been nominated by his peers as player of the year.
However there been much speculation that giants Real Madrid may try and lure him back to his homeland.
And Neville says the Reds hierarchy must rebuff any overtures, and tie him down to a new deal.
The treble winner says he has been left stunned at the youngster's transformation after joining from Athletico Madrid for £17.8 million in 2012.
Neville said: "The biggest thing is David De Gea, and keeping him at the football club.
"From when he first came, to how he is now, I don't think I've seen a player develop and improve as quickly as him.
"I saw with Cristiano (Ronaldo), I saw the development in terms of his performance, his size, his stature and his physique.
"He's now at the stage where you'd say he's world-class.
"He's winning football games on his own. It's a flippant thing to say but he really has.
"Last minutes saves against Everton, Hull, Arsenal away - there's seven or eight occasions where he has won United games which just shows you that he is world-class.
"And if they can hold on to the shot stopper, Neville says his old side can go all the way next term."
Neville added: "I think what has happened - the performances at Anfield and Stamford Bridge I think have let the whole Premier League know Man United will be a force next season.
"I don't think just finishing in the Champions League won't be the aim.
"I think from day the words coming out of the mouth of Louis Van Gaal, and the fans, will be 'we want to win the Premier League title back.'
"It's okay saying those words but I think the end to this season has proved that that will be achievable."
Neville, meanwhile, believes Marouane Fellaini will enjoy his return to former club Everton when the Reds visit Goodison Park on Sunday.
The Belgium international was an unused substitute in the fixture last year - a 2-0 defeat which turned out to be David Moyes' final game in charge of the Reds - but can expect to be a key figure this time around after earning the faith of manager Louis van Gaal.
"I think it'll be good for Fellaini to go back to Goodison now because he's going back in a better of state of mind," the ex-England international said. "He's in better form and I think he's going there full of confidence.
"What happened in terms of his departure from Everton has been forgotten about now so Everton fans can probably look at it and say Marouane did a great job for us, he's coming back and United fans are seeing the best of him. So he's got to go there enjoy the fact he's playing really well for the club now and he had five great years at Everton too."
United are aiming to bounce back from the defeat at Chelsea by completing a double over the Merseysiders and maintaining the push for a top-four spot.
"I'm really looking forward to the game," Neville added. "I'm actually going to go to it because it's two of my former clubs. Everton have come back into a little bit of form while United are probably the form team in the Premier League at the moment so it should be a really open game as it's two coaches with a real focus on a tactical battle."