The Red Devils are optimistic about their chances of signing Bale, but now fear Real will demand De Gea as part of any deal that sees the Wales star move to Old Trafford.
On United's pre-season tour of the United States, Mourinho whispered a cheeky message to Bale as he greeted players before his side lined up against the European and Spanish champions.
He said: “I cannot buy you because you do not talk to me,” making it clear to the former Spurs star that he is a wanted man.
Bale is not having the best of times in the Spanish capital. He has been jeered by Real supporters and recently picked up another injury, leading to speculation suggesting Zidane is ready to cut his losses on the pacey forward.
Meanwhile, De Gea is happy in Manchester, but has reportedly told close friends that playing for 'Los Blancos' is on his career bucket-list.
That leaves Mourinho in a very tricky situation. The Portuguese tactician is a long-term admirer of Bale, but will he be willing to hand over one of the best goalkeepers in the world for the 28-year-old's services?