City face Real in the Champions League a week on Tuesday.
Asked if he was worried about that prospect, Mourinho said: "We have the potential and history to keep our best players.
"Madrid, Barca, Bayern, these clubs have unique historic capital – the trophies, the European Cups, the history, the fans. Things you cannot buy with money.
"The best players will want to be at these clubs, the three I have mentioned and some others who have trophy cabinets full for almost a century."