The controversial decision prompted much debate, especially after Real went down 3-2, with back-up goalkeeper Antonio Adan taking Casillas' place.
Mourinho claimed the decision was "purely technical" and insisted he did not regret dropping the Real captain despite defeat leaving them 16 points behind Barcelona.
Casillas has been only professional and discreet about Mourinho's decision and on Wednesday told reporters he is just determined to regain his position.
He said: "I'm training myself to fight, to fight and to recover the position. It is the goal that I have and that the coach selects me in the next game."
The Spain international added: "I feel good but I'm not a machine, I have to train every day and there are better and worse times, but I feel fine.
"Above Casillas is Madrid. The decision is the coach's.
"In any case, the day I no longer have the desire to work, I'll stop."