The duo are far from friends following their time at Real Madrid when the manager dropped the Bernabeu legend in favour of Keylor Navas.
Casillas has already revealed his regret at not confronting his former boss while Mourinho fired back with another sensational comment about the keeper reaching the end of his career.
But it appears the 37-year-old Porto keeper wants to have the last word following United's dismal defeat at Liverpool.
It left Mourinho's men a staggering 19 points behind Liverpool with the Portuguese boss clinging onto his job.
And it appears Casillas is revelling in Mourinho's failure at Old Trafford.
He tweeted: "According to a Portuguese newspaper someone said that a player like me (37 years) is at the end of his career. Completely agree!
"My question is, for that newspaper: in case of the coaches, when and at what point do you see that they are no longer to lead a team or train?"
United legend Gary Neville also stuck the knife into Mourinho and claims he will be sacked soon.
Speaking to Sky Sports, Neville said: “Will Mourinho leave? I think it'll happen. My preference would always be to get to the end of the season. But the boardroom is so naive it’s unbelievable,
“To give him an extended contract, knowing his cycle of every three years, was incredible. The minute he came back from pre-season he was at it and the club was out of control.
"18 months he's 2nd in the league, then you need to keep him hungry.
"His agent was out last week, at press conferences they don't know what he is going to say next, and they don't know what to do with him."
"The problem they will have now is he would cost an absolute fortune to lose him, when you lose a manager in the season you have the situation of who comes in?
"It is not just as easy as saying 'let's get rid of the manager' because Manchester United need a reset and it is not just the manager. It's deeper than that."
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