David de Gea
David de Gea's contract at Old Trafford is due to expire in summer 2016. Getty Images

Former Manchester United and Everton star Phil Neville has insisted Louis van Gaal's most important business ahead of the summer transfer window will be to keep David De Gea at Old Trafford rather than adding to the current squad.

The Spanish goalkeeper, whose current contract at the Premier League giants is due to expire in summer 2016, has been linked with a move to Real Madrid in recent months amid the uncertainty surrounding the future of veteran Iker Casillas at the Santiago Bernabeu.

Van Gaal recently revealed United had already offered De Gea a renewed deal to commit his future to Old Trafford reportedly worth in the region of £200,000-a-week.

Asked which positions United should bolster during the summer transfer window, Neville said to the Manchester United official website: "My biggest fear in the summer, I have to be honest, is over a player who is currently in the club like David De Gea. I want him tied up on a long-term contract.

"He is the best developer I have seen for a number of years. From when he first signed to where he is now, the development and the improvement he has shown has been nothing short of incredible," he added.

"He is the best around. He has won Manchester United games. He is up there with (Edwin) Van der Sar and (Peter) Schmeichel in terms of winning games so that would be my one worry this summer."

De Gea was recently named as the number one in the PFA team of the year ahead of Chelsea keeper Thibaut Courtois. He is also one of the favourites to win the United players of year award.