Mourinho called Real's performance "unacceptable" after his team squandered a halftime lead to lose 2-1 at their city rivals on Sunday. The league champions are without a win in three matches in all competitions this season.
They must overturn a 3-2 deficit in the second leg to win the Super Cup and Mourinho told a news conference he wanted to see a complete change in attitude at Real's Bernabeu stadium.
"For me the only problem is if the difference in performance between the first two games and the last match remains, that's the only problem," he said.
"Win the Super Cup, not win the Super Cup. Beat Barca, not beat Barca. None of that is important to me right now.
"My priority right now is to know whether the attitude we had against Getafe was a chance happening or whether it's a situation that has had consequences on the psychological profile of some players.
"That for me is the fundamental thing for tomorrow."
Attention was deflected on Monday from Real's poor start to the campaign by blanket media coverage of the signing of Luka Modric from Tottenham Hotspur.
The Croatia playmaker trained with his new team mates on Monday evening and Mourinho said he would be ready for action soon despite not playing a competitive match this season and training apart from the rest of the Tottenham squad.
"Modric's condition is normal for a player who hasn't trained with their team in a while," Mourinho said. "A player like Modric will be in a good form soon."
The Portuguese manager did not rule out more arrivals and departures before the European transfer window closes on Friday.
Central defender Pepe, who suffered a head injury in the 1-1 home draw with Valencia on the opening weekend of La Liga, trained as normal on Monday and will hopefully be available to play against Barca, Mourinho said.
Barca have won the Super Cup, which pits the league champions against the King's Cup winners, for the past three years. Real last triumphed in 2008.