Casillas and Mourinho worked together at the Bernabeu.
However, during his three-year stint in Madrid, Mourinho fell out with several influential stars – including Casillas.
The experienced goalkeeper was frozen out by the current Manchester United boss and was publicly ostracised.
Casillas has criticised the Portuguese’s man management and his playing style.
He told Bein Sports: “I also believe that the first year of Mourinho we were all in a bubble because of the rage.
“We won that Cup final that was very beautiful and that people even lived like a Champions – but we did not know how to finish with the Champions.
“The third year with Mourinho was not very good.
“We had already had friction in the second year and the situation was quite delicate.
“The third year was traumatic, also with an injury in between.
“There were too many people around who began to shake the relationship and it was not good for him, not for me and what is more important, not for Real Madrid either.
“I did not like what I saw anymore, what people talked about.
“I know very well how to speak in the field of Madrid, I grew up here, I have been captain, I have been with references in the world of football,”
Casillas, who is now strutting his stuff at Porto, laments not being more confrontational with Mourinho.
The 37-year-old also admits his relationship with his former boss has not recovered.
He added: “Honestly, I tell you now, if I were to happen again I would have taken the bull by the horns and I would have faced him.
“But at the time I thought that the best thing was to keep quiet and not respond.
“They were stinging me to jump and I did not want to.
“With Mourinho he had had many discussions, good and bad, and we knew each other very well.
“None of us have talked about each other again.
“We prefer to leave it at ease, when we saw ourselves in Porto we said hello, in the World Cup… better let it go.”
Mourinho was involved in a high-profile spat with Paul Pogba earlier this season.
After Pogba was stripped of the vice-captaincy, the duo were involved in a frosty training ground row.
The ex-Chelsea manager has also publicly called out Anthony Martial.