David de Gea’s Manchester United future was thrown into doubt on Thursday night as his agent failed to rule out a move to Real Madrid in the summer.

De Gea has 18 months left on his contract and has yet to agree new terms to stay at Old Trafford as he takes stock of his future with Real watching developments.

His agent Jorge Mendes, who also represents Cristiano Ronaldo, was asked by Spanish media if he could see the 24-year-old moving to Madrid.

Agent Jorge Mendes has not ruled out the possibility of David de Gea joining Real Madrid

De Gea has been in outstanding form for Manchester United during the course of the season

Mendes, pictured with Radamel Falcao, has not ruled out the possibility of De Gea leaving Old Trafford

'David is amazing, he’s brilliant,’ the Portuguese agent told Marca. ‘He’s a Man United player and has a contract there. You have to respect that but things change every five minutes.’

Despite the signing of former Barcelona keeper Victor Valdes, United boss Louis van Gaal has insisted that De Gea will extend his current deal at Old Trafford.

Van Gaal, whose side face Cambridge United in the FA Cup on Friday, dismissed criticism of his tactics and claimed that United’s traditional 4-4-2 formation leaves him nervous on the bench.

After discussing the merits of a back three over a back four, he insisted he would ignore critics who want him to ditch his favoured 3-5-2 line-up.

In a variation on Sir Alex Ferguson’s famous quote about ‘squeaky bum time’, Van Gaal said: ‘I know that with 4-4-2 the stats are giving the positive results, but when you analyse the game I have twitched my a*** on the bench because we were out of balance — I don’t like it.’

Louis van Gaal believes De Gea will sign a new deal to stay at Old Trafford despite Real Madrid's interest