The former 'keeper has also highlighted the criteria any potential replacement must meet.
The Spanish goalkeeper seems set for a return to La Liga with Real Madrid this summer with Rafa Benitez’s side having been linked with him for quite some time by the likes of the Daily Mail.
And with just a year left on his contract, United would prefer to get a good price for their star man this season rather than losing him on a free next year.
While it would be a blow to manager Louis van Gaal to lose his undisputed number one before the season starts, Van der Sar believes the 'keeper can be replaced.
“Nobody is irreplaceable, that's always the case,” he said.
“But I'm sure United, if David is going to leave, are going to find a good successor. A new player may need a little bit of adjustment or will take some time to see how the Premier League works and the different type of football.”
If De Gea does eventually leave Old Trafford at some point, United will likely have to enter the transfer market to find his replacement, which could spark a summer transfer merry-go-round of goalkeepers.
A new signing may not be necessary though as Van Gaal can look to current number two Victor Valdes should the need arise.
Regardless of who it is that would become the new number one, Van der Saar believes there are certain criteria that must be met.
“You have to ask yourself 'what are the criteria Louis van Gaal needs for his goalkeeper?” he added.
“David has immense reflexes and is also good at coming out for corners. He's improved a lot there. Louis also likes a goalkeeper who can play with his feet also and just behind the defence.”