De Gea has frequently been linked with a return to Spain's capital, although the stopper has recently claimed that he remains "very happy" at Old Trafford.
However, United are aware that Real could still come calling during the summer and according to Don Balon, Mourinho has suggested that he will only contemplate a deal if Bale returns to the Premier League.
Injury has restricted the Welsh star to just 11 starts and four substitute appearances this season, but the 28-year-old has still managed five goals since the start of 2018.
Reports have previously suggested that Real are open to selling Bale in the summer, and it appears that Mourinho will look to hand the attacker another chance to play in England's top flight.
Bale still has four-and-a-half years left on his contract at the Bernabeu.