Mourinho said a Roma player had shown a poor attitude during their 1-1 Serie A draw at Sassuolo in November. He did not name the player but Italian media reported that it was Karsdorp to whom he was referring.

“Karsdorp has been accused in public of being a ‘traitor’ – a hurtful and baseless term that the club management has failed to address or apologise for, and fans have repeatedly used to confront him and his family,” Fifpro said in a statement.

“Furthermore, he has been made the subject of an unjustified disciplinary case. These acts apparently are a way to deflect from the club’s poor recent performances and aimed to put unjustified pressure on the player.

“Such behaviour is incompatible with the Fifa regulations.”

Roma, who sit seventh in the Serie A standings, won only one of their last five league games and resume their Serie A campaign at home to Bologna on Jan 4.

“Karsdorp has the full support of both Fifpro and the Dutch players’ union VVCS,” Fifpro said.

Fifpro general secretary Jonas Baer-Hoffmann said clubs must take care of their players.

“We call on AS Roma to immediately comply with that duty of care and ensure that Rick Karsdorp is treated fairly, and that his rights are respected,” he said.

Roma were not immediately available for comment.