The La Liga champions want to bring De Gea back to Spain as a long-term replacement for the 31-year-old Casillas, and they're prepared to sanction a move regardless of Jose Mourinho's future at the club.
The Madrid boss has been tipped to leave the Bernabeu at the end of the season, and the report claims the defending champions will look to sign De Gea even if the 'Special One' does call time on his Real Madrid career.
Casillas has been Real Madrid's number one since his teenage years, but the Spanish club are planning for the future.
The Spain shot-stopper has been injured this season, and Real Madrid have scrambled to recruit Diego Lopez as a short-term stop-gap.
But they're unlikely to want to fall into the same situation again, and the report claims they have already begun planning for next season.
22-year-old De Gea is highly-rated in Spain - national team boss Vicente Del Bosque has indicated the former-Atletico man is in line for a call-up - despite heavy criticism in England.
De Gea turned in his best performance in a United shirt so far at the Bernabeu last week, as United held Madrid to a 1-1 draw in the first-leg of their last-16 tie.
But his errors earlier in the season have been picked apart by some who feel that the young keeper is not cut out for the physicality of the Premier League.
Reports have suggested United are ready to cut their losses on their £17m signing, and they've reportedly lined up Stoke shot-stopper Asmir Begovic as their ideal target in the summer.