The former Real Madrid star and current FC Porto goalkeeper took to Twitter after United's 3-1 loss to Liverpool on Sunday, and while he didn't mention Mourinho by name, AS' Robbie Dunne seems to have deduced who he was talking about:
Just last month, Mourinho took a shot at Casillas without mentioning the Spaniard by name, suggesting someone was orchestrating a media campaign against David De Gea, per James Ducker of the Telegraph.
The two have been at odds for years, but things had gone mostly quiet until this season. They took shots at each other in interviews in November, per Football365:
Per Ducker, their feud dates back to the time they spent together at Real Madrid.
Mourinho dropped the club legend from his starting XI and replaced him with Antonio Adan. The veteran stopper was accused of being a mole for the press, and fans eventually turned against the then-Spain international, per Sky Sports' James Walker-Roberts.
He would depart for Porto in 2015, and many thought his career was all but over, but the 37-year-old has rediscovered his form in Portugal and has been among Spain's top goalkeepers for the last two years.
As Ducker reported, he has made no secret of his desire to return for the national team, and a recent AS poll revealed many fans are in favour, per Marco Gonzalez.
Mourinho's United already sit 19 points behind rivals and Premier League leaders Liverpool in the standings and have a goal differential of zero after 17 matches, scoring and conceding 29.
His time at Old Trafford appears to be coming to an end, and according to the Mail on Sunday's Oliver Holt, it's not the hope of improvements that is saving him right now:
United's next match will be at Cardiff on Saturday.