The La Liga club played nine games in January, five in La Liga and four in the King's Cup, and a 1-1 league draw at Almeria and Sunday's shock 1-0 defeat at Osasuna left them seven points behind leaders and great rivals Barcelona.
"Playing nine matches in a month is not a problem for players who are used to it," Portuguese Mourinho told a news conference ahead of Wednesday's Cup semi-final second leg at home to holders Sevilla, for which Real have a 1-0 advantage from the first leg.
"But these (Real players) are not because for several years now Madrid has been eliminated from the Cup in the first or second knockout round and in the Champions League the same.
"They are not used to it due to bad lack or circumstances but the fatigue is not physical because players of this calibre are ready to play a match every two days.
"From a psychological point of view it takes more getting used to."
Real are bidding for a first domestic cup since 1993 and if they advance to the final will almost certainly meet Barca. Pep Guardiola's side thrashed Almeria 5-0 in the first leg at the Nou Camp and also play their second leg on Wednesday.
The final is on April 20, a few days after Real host Barca in La Liga for the second "Clasico" of the season.
The La Liga giants could also be drawn to play each other in the Champions League quarter-finals if Real get past Olympique Lyon, who knocked them out at the same stage last season, and Barca eliminate Arsenal.
CREATURES OF HABIT
Mourinho said the experience of playing so many games over such a short period would stand his players in good stead for the future.
"I hope they will be better next year and even better in two or three years because the goal is to play in all competitions," he said.
"These players are human beings and there are human beings in other teams who play every three days.
"The human being is a creature of habit so that means they will be better used to playing more regularly next year."
