Casillas, who worked under Mourinho during the Portuguese manager's three-year spell at the Bernabéu between 2010 and 2013, posted his views on X in response to growing reports that Mourinho is set to return. He was direct.
"I don't want him at Real Madrid. I think other coaches would be better suited to manage the club of my life."
He stopped short of making the objection personal, acknowledging Mourinho's quality while making clear the opinion is about fit rather than character.
"He seems to me a great professional. I have no problem with Mourinho. Personal opinion. Nothing more."
His broader concern was about the current direction of the club.
"When Real Madrid goes two seasons without winning a title, it is something to worry about."
The comments generated significant backlash from a section of the fanbase, prompting Casillas to note the double standard he observed when former Real Madrid and Germany midfielder Toni Kroos made near-identical remarks without facing the same criticism.
"My opinion — which copies his to 99 per cent — is attacked by a certain sector of a 'dark' section of Real Madrid supporters. Astonishing, this world of social media."
Casillas made 725 appearances for Real Madrid between 1999 and 2015, winning five La Liga titles and three Champions League trophies. The relationship between him and Mourinho during their three years together was famously tense — the manager publicly questioned his position as first-choice goalkeeper on multiple occasions.
Reports from Sky journalist Sacha Tavolieri have since claimed Mourinho has already reached an agreement with Real Madrid's leadership. Casillas's rejection of the appointment has not changed the direction of travel — but it is a significant public intervention from one of the club's most iconic figures.