The 27-year-old was close to joining Real in August 2015, but the move fell through in the final hours of the transfer window as the paperwork was not completed in time. In January they were on the verge of inking Athletic Bilbao keeper Kepa Arrizabalaga before he signed an extension.
But Mourinho has warned his former employers off pursuing De Gea.
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That progress will come to a shuddering halt if De Gea's contract is allowed to run down to just one year remaining and his future will become an increasingly debated topic over the next few months if he doesn't sign a new deal at United.
De Gea has enjoyed another stellar season for United, keeping a Premier League-best 15 clean sheets this term to help Mourinho's men occupy second position behind runaway leaders Manchester City.
"He's world class, some of the saves he's made".
The first leg in Spain finished in a goalless draw, such that it's all to play for heading into Tuesday night's game in Manchester.