Sweeping changes at Old Trafford are being mooted as United get ready to embark on a second fresh start in as many years.
David Moyes' tenure proved to be an ill-fated one, with the Scot relieved of his managerial duties after just nine months.
He did, however, snap up Spanish playmaker Mata from Chelsea in a big-money swoop in January, proving that United remain a big draw to top talents.
De Gea believes it is imperative that such 'important' figures are kept on by Van Gaal when he inherits the reins after the World Cup, even if, as expected, United splash the cash again in an effort to fire themselves back into the Premier League title picture.
The Spain international goalkeeper told Movistar: "I would not sell Juan Mata. He is a very important player for us.
"We have taken a qualitative leap with him and he has adapted very well."
Mata's position has been called into question in the wake of reports linking United with another attempted raid on the Barcelona ranks.
Cesc Fabregas has been a top target for some time, while World Cup winner Andres Iniesta is also reported to figure prominently on Van Gaal's transfer radar.
De Gea believes both would be welcome additions, with Iniesta in particular having shown over a number of years that he is one of the finest creative midfielders in the business.
"If I had millions and millions, I would buy Iniesta," said De Gea.
"He is one of the best players in the world. He has character, quality and goals. He has everything."