Mourinho - who declared his interest in Bale on Monday - hands competitive debuts to new signings Nemanja Matic, Romelu Lukaku and Victor Lindelof against his former club. Paul Pogba is also in from the start against the European and Spanish champions.
"If he (Bale) is not in the club's plans, that with the arrival of another player would mean he was on his way out," Mourinho had said.
"I will try to be waiting for him on the other side and fight with other coaches that would want him on their team. If he's playing tomorrow, I wouldn't think of that, it would be because he's in the coach's plans and the club's plans, because he also has the motivation to continue at Real Madrid."
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But Madrid coach Zidane opts to go with Bale from the outset as Cristiano Ronaldo takes his place on the bench as he returns to fitness following his exertions with Portugal at the Confederations Cup.
Manchester United: De Gea, Valencia, Lindelof, Smalling, Darmian, Matic, Herrera, Pogba, Mkhitaryan, Lingard, Lukaku..subs: Romero, Blind, Carrick, Fellaini, Mata, Martial, Rashford
Real Madrid: Navas, Carvajal, Ramos, Varane, Kroos, Benzema, Modric, Bale, Marcelo, Casemiro, Isco..subs: Casilla, Nacho, Ronaldo, Hernandez, Vazquez, Asensio, Kovacic.

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Phil Jones will miss out for United despite ongoing appeal against his suspension - a two-match ban that could yet be extended by a game.
UEFA recently handed the 25-year-old defender Jones the ban for verbally abusing a doping control officer after being selected for testing after May's Europa League triumph against Ajax.
The Super Cup is the only European trophy Mourinho is yet to win as a coach, having lost the 2003 and 2013 editions while with Porto and Chelsea respectively.
United have not lifted the trophy since 1991, and the Portuguese coach said his side were the underdogs to double Champions League holders Real.
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