Reports in English media claim Mourinho's recent behaviour, complaints about transfers and man management has caused strain in the dressing room.
His criticism of the club's high-profile players throughout the 2017/18 season and now the youth during pre-season also raised a few eyebrows within the club.
According to the Daily Mail, the Red Devils hierarchy are preparing a succession plan for the Portuguese tactician, who's never remained at a single club for longer than a period of three seasons.
And as he enters his third season with the 13-time Premier League champions, the pressure is mounting for the 'Special One' to bring silverware to the iconic club - something which eluded in the previous campaign.
Zidane ended his time at Real Madrid days after securing a third straight Champions League triumph and is currently out of work heading into the new season.
While he was previously tipped for a sensational move to Juventus, nothing came of the rumours and the Frenchmen is currently 'enjoying a sabbatical from the game'.