Phil Neville fears David de Gea will leave Manchester United this summer.
European champions Real Madrid have been monitoring De Gea's contract negotiations with United and are ready to pounce if the Spaniard does not sign an extension to his current deal, which expires at the end of next season.
A fortnight ago manager Louis van Gaal said negotiations had been ongoing for "a long time", but De Gea has yet to commit his future to United despite being offered "a lot of money", in the words of his manager.
And former United left-back Neville admits the club could lose their star goalkeeper at the end of the season.
"My biggest fear in the summer is David de Gea," Neville told United's official website.
"I want him tied up on a long-term contract."
Although his form has tailed off slightly of late, De Gea has enjoyed a fine season for United and was named in the PFA team of the year last month.
Few would have thought De Gea would turn into such a formidable goalkeeper at the start of his time at Old Trafford when he made a number of costly errors.
But four years on from his debut, the former Atletico Madrid stopper now ranks among the greatest goalkeepers in the club's recent history, according to Neville.
"He is the best developer I have seen for a number of years," Neville said.
"From where is when he first signed to where he is now, the development and the improvement he has shown has been nothing short of incredible.
"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 is the overwhelming favourite to win United's player of the season award, which he also won last year.
The Spain international believes he is yet to reach the peak of his powers despite two impressive seasons at Old Trafford.
"Yes, I have (improved this year)," the 24-year-old said.
"I'm very young and I always try to keep getting better, in every training session, every match, and I think it's been a very productive season for me. I'm still growing as a player; I'm very satisfied with the season that I and we have had."
Neville, meanwhile, has revealed he is looking to go into management. The former left-back was United first team coach under David Moyes last season, but left the club last summer following Van Gaal's appointment as manager.
The 38-year-old has been taking his coaching badges while working as a BBC pundit and hopes to find a suitable club in the near future.
"Yes, it's something I want to do," Neville said when asked if he wanted to go into management.
"I am taking my (UEFA) Pro Licence, which is the last qualification. I am learning. I had a fantastic learning experience as first-team coach at Man United. You can do all the training courses in the world but the experiences I learned last year - it's something you can't replicate."