Why would Zidane be any better than Mourinho for United? Zidane had peak Ronaldo during his Real tenure, peak Bale, Ramos, Modric, Benzema, Varane, Marcelo too.
My Nan could manage Real and have a good season. United with their current squad is a much harder job. The ghost of Fergie hangs over whoever takes over.
Rafael Benitez had the same players but his performance was so poor that he was sacked and I believe that his sacking was what brought Madrid their golden Era. Not everyone can manage superstars successfully.