Casillas criticised Mourinho for creating a war-like atmosphere between eternal rivals Real Madrid and Barcelona.
And he admitted during an interview with former Argentina international Jorge Valdano he regretted staying silent and would now 'take the bull by the horns and confront' Mourinho if he could have his time again.
Mourinho, who dropped Casillas halfway through the 2012-13 campaign after accusing him of leaking stories to the press, reacted by branding the shot-stopper as someone who is in the twilight of his career.
He said of Valdano's TV interview with Casillas, who left Real Madrid for Porto after being sidelined: 'It looks like an interview with someone who is at the end of his career.
'When he says he never confronted me he's not telling the truth.'
In quotes published by Portuguese sports daily Record he added: 'He confronted me and he did it in a way in which no-one does it better than him, secretly.'
The 37-year-old Spaniard, who made his first-team debut for boyhood club Real Madrid aged just 18 and was dubbed Saint Iker for his spectacular saves, has admitted he has no contact with the United manager.In recent weeks he has responded to the question marks over David de Gea's performances for the Spanish national side by admitting he would jump at the chance of an international recall.
Former coach Julen Lopetegui left him out of the Spain squad for the 2018 World Cup and new manager Luis Enrique has yet to call on him so he can add to his record 167 caps.
He told fans in October, after De Gea's perceived poor showing in Russia where Spain went out against the World Cup hosts: 'I understand Luis Enrique has new ideas for this new era, but I am still hopeful of a return.'