De Gea was earmarked by manager Sir Alex Ferguson as the veteran Dutchman's successor when he retired last summer, securing De Gea's signature from Atletico Madrid and handing him the number one shirt.
However the Spanish keeper has struggled to live up to his billing so far this season, producing a number of nervous displays that saw him replaced by rival shot-stopper Anders Lindegaard.
The Dane recently suffered an injury in training, meaning De Gea was restored to the starting line-up for Sunday's thrilling 3-3 draw with Chelsea, where he pulled off a sprawling save in the dying minutes to keep out Juan Mata's free-kick.
And Van der Sar says it will take time for the youngster to adjust to both his new surroundings and the pressure of his big-money move - something he must do quickly.
"It's always difficult if you're coming into a new club and new league, especially the English one. It's a very demanding one, can be a very tough one. He has to deal with that. They paid a lot of money for him and he has to come through that," he said, as reported by The Guardian.
"I spoke to Anders and he's disappointed of course, that he's injured at the moment because he's really been magnificent."
Van der Sar insisted that he was not about to copy Paul Scholes and come out of retirement to help solve the goalkeeping crisis at Old Trafford.
"I got a few text messages from friends saying: 'What about you?'," he said. "I'm quite happy at the moment. There's no doubt in my mind."
