The defensive midfielder has been linked with a move out of the Santiago Bernabeu, with the Barclays Premiuer League giants reportedly willing to fork out £14 million to acquire his services.
United boss Sir Alex Ferguson wants to inject more steel into his midfield and sees the former Arsenal, Chelsea and Portsmouth midfielder as the ideal signing.
But the Scot's hopes of landing the 25-year-old have been dashed by his former nemesis.
Mourinho was quoted as saying in The Daily Mail newspaper: "Diarra has played well recently. He was even in the starting 11 while usual midfielders were available.
"That proves I want him to go on here."
This latest development comes after news that Diarra wanted a move away from Madrid due to the lack of first team football since the arrival of Sami Khedira last summer.
Diarra joined Madrid from Portsmouth in 2009 for a fee of £20 million.