The Portuguese tactician has warned the La Liga giants, suggesting them to "think of another player" instead of the 27-year-old.
The Spanish goalkeeper has been consistently linked to the Los Blancos. Real all but landed their man in 2015 after they agreed to a £29.3m deal with United for De Gea and Keylor Navas moving from the Spanish capital to Manchester.
However, the deal fell through due to issues related to paperwork ahead on the transfer deadline day. Even as Real accused United of the delaying the completion of the paperwork, De Gea went on to sign a four-year contract extension with an option of one-year extension.
"I think he will stay. I don't know what Real Madrid's intentions are, but if I were them, I would think of another player," Mourinho told Spanish television channel La Sexta, as quoted by goal.com.
De Gea's current contract with United will come to end at the end of next season, but Mourinho had said in January 2018 that the Red Devils will definitely trigger the extension clause.
"It is obvious that we are not going to let the option [contract extension] disappear. A goalkeeper like he is, at a club that wants to be better and better and better – we are not going to let that option go away. We are going to try," Mourinho was quoted as saying by Sky Sports.
De Gea, who is reportedly earning £210,000 a week, will demand at least £375,000 a week when United approach him for a new deal, Manchester Evening News reported in February 2018.
The ever-consistent Spanish goalkeeper has been on top of his game in the ongoing season, and United have managed to cling on to the second spot on the Premier League table with 65 points from 30 matches, 16 points behind leaders and cross-town rivals Manchester City.
Unsurprisingly, De Gea, who is also on Ligue 1 giant Paris Saint-Germain's radar, is on top of the list of goalkeepers with maximum clean sheets in the league season — 15.
He helped United secure a goalless draw in their Champions League Round of 16 first-leg tie at Seville last month. The Premier League giants will take on Sevilla at Old Trafford on Tuesday, March 13.