After taking over a struggling Real side, Mourinho helped them challenge the dominance of Pep Guardiola's Barcelona, winning La Liga, Copa del Rey and Spanish Super Cup trophies in his first two seasons at the Bernabeu. The 100 points they amassed in 2011-12 remains a La Liga record.
However, the wheels came off in Mourinho's third season in the Spanish capital and he was dismissed at the end of 2012-13 with Real finishing 15 points behind rivals Barcelona.
His treatment of Casillas divided the dressing room. The goalkeeper made 725 appearances across 25 years at Real, winning three Champions League titles and five La Liga crowns, but had spent much of the 2012-13 season on the bench.
"The relationship Jose and I had was very cold," he said.
"In those years, I really think it was taken to a limit where in the end, the rope broke."
Former Real midfielder Sami Khedira said that Mourinho's rift with Casillas affected everyone.
"Iker is a Real Madrid legend, loved by every Real Madrid supporter. He benched him, doesn't speak to him, spoke bad about him in the media, so then this locker room was a nightmare, honestly," he said.
"After that, the relationship between Jose and many, many players was broken – like Cristiano \[Ronaldo\] and the Spanish guys, Casillas, Sergio Ramos. And those players were super, super important for the atmosphere in the locker room."
Mourinho claimed that the keeper had a bad attitude.
"The first three times that I spoke to Casillas, the captain, the first thing he told me was, 'I want to ask for more holidays for the national team players'," he said.
"The second time he spoke to me was, 'I would like to ask you to put the training sessions one hour later because at the time you want the training sessions, there is a lot of traffic in Madrid'. The third time he comes to speak with me was, 'We don't want to go to \[the\] hotel. We prefer just to meet on matchday and go direct to the stadium to play'. And in a short period of time, I realised they're spoilt."
Former Spain captain Casillas also revealed that he clashed with Mourinho over the manager's attitude towards his international team-mates who played for Barcelona.
The 63-year-old's response was unapologetic.
"There are things that I don't accept and I will never accept and when somebody doesn't respect me, it's a problem. I say, this is Barcelona, Real Madrid. When you go to the national team, then you can kiss the guy, but not now. This is war."