The European champions have been long-term admirers of De Gea and came close to signing him in 2015 only for an admin error to scupper the deal just before the deadline.
Madrid are expected to revive their interest in the Spain number one this summer as they look to rebuild their squad following a disappointing campaign which sees them sitting 19 points adrift of leaders Barcelona in La Liga.
Spanish outlet El Confidencial reports that Los Blancos would even be willing to use five-time Ballon d'Or winner Ronaldo in any approach for De Gea.
Ronaldo has been linked with an exit from the Bernabeu at the end of the season having reportedly grown disillusioned with life in the Spanish capital, and a return to Old Trafford is thought to be an attractive proposition for Madrid's all-time leading goalscorer.
However, United manager Jose Mourinho would be reluctant to accept such a deal as he values De Gea higher than even Ronaldo, who has scored 426 goals in just 420 games for Madrid.