David De Gea
David de Gea has been constantly linked with a move to Real MadridGetty

Manchester United will struggle to keep hold of David de Gea if Real Madrid make a firm bid for the goalkeeper, according to Liverpool legend Jamie Carragher.

The Spain international's contract at Old Trafford ends in 2016 and he has not agreed to a new deal. Red Devils manager Louis van Gaal remains confident that the 24-year-old will agree to sign a new deal with the 20-time English champions.

United saw Cristiano Ronaldo swap Old Trafford for Santiago Bernabeu in 2009 as they were unable to retain the Portuguese international. The former Liverpool star believes the Premier League giants will not be able to stop the Madrid-born keeper in returning to Spain, if Real decide to sign De Gea.

"When Real Madrid come in - no matter who you are - they can still take them," Carragher said, as quoted by the Manchester Evening News.

"They did it with Cristiano Ronaldo here. Unfortunately as he's Spanish it may be that dream move for him one day. It was the same with Xabi Alonso at Liverpool."

"The worry for Louis van Gaal may be how close is De Gea to signing a new contract. That maybe in the back of his mind," he added.

Van Gaal has already taken precautionary measures as the Dutchman signed free agent Victor Valdes on an 18-month deal last week. The former Barcelona keeper is United's second choice goalkeeper an was on the substitute's bench during their 1-0 defeat to Southampton.

Carragher believes Valdes will struggle to get regular playing time as long as De Gea is at United.

"The start of next season Victor Valdes - due to his reputation and what he's won playing at Barcelona - will want some game time," the Anfield favourite said.

"He may get that as we expect Manchester United to be in the Champions League. So he will have more games to play, but I don't see him ever overtaking De Gea."

"They have problems defensively, but they don't concede that many goals because of him. Their defensive record is quite good."

"The manager has come in and every manager will want a good back up goalkeeper. He's maybe looked at Anders Lindegaard and thought he's not the answer," Carragher concluded.

Should De Gea join Real, United will use him to sign, the Spanish capital club's star player Gareth Bale. The Welshman has been constantly on Van Gaal's radar as the former Bayern Munich manager looks to add the forward to his star studded squad.