The Madrid-born Spaniard has been at the centre of the most intense transfer saga of the summer, with La Liga giants Real keen to bring him in to replace legendary shot-stopper Iker Casillas.
Sergio Romero, who was chosen ahead of De Gea for the Premier League opener against Spurs on Saturday, represents United's next-choice No 1, with Sam Johnstone and Anders Lindegaard also vying for a place. United, and Louis van Gaal in particular, remain keen to keep hold of their star, with one year remaining on his deal at Old Trafford.
Reports had claimed last month that Real could wait until then to sign the 24-year-old for free, but rumours linking Rafael Benitez with a big-money move this summer have since resurfaced, and Ferdinand believes fourth-favourites for the title United would be throwing away a potential title challenge with his sale.
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"If Manchester United lose David De Gea they can forget about a title challenge," he wrote in his column for The Sun. He added: "A club like United cannot allow a keeper of De Gea's stature to leave because the transfer fee they receive from Real Madrid will not make up for missing out on major trophies. Should David choose to walk out the door next summer, when his contract expires, then so be it. But United don't need the cash and they can afford to be strong. I know De Gea well from my days playing with him at Old Trafford and he is too professional to throw the towel in and sulk. He might not like the decision, but it's not the biggest hardship in the world to have to play for Manchester United for another year."
Manchester United legend Ferdinand, once the club's record signing, says that only a potential swap deal for a Real Madrid player would appeal to him. "The only way I would even think about selling him is if Gareth Bale, Karim Benzema, Sergio Ramos or Raphael Varane came the other way and it's clear that's not going to happen."