The player’s identity, who has not been revealed but who is seen as loyal to Mourinho, is said to have told pals that almost every Manchester United player no longer backs the manager.
The Sun add that there are backroom staff who have fallen out with the Portuguese boss too.
They are confident they will outlast him at the club with morale falling so low that there are fears Liverpool could thrash them at Anfield today.
The Sun report that one member of staff, who had a run-in with Mourinho, said: “I’ll be here long after he’s gone.”
Super agent Jorge Mendes issued a statement earlier this month claiming his client Mourinho was going nowhere.
But Express Sport understands Mourinho is on a collision course with his bosses over the club’s lack of transfer action.
It’s believed the United boss is seething no deals are being worked on behind the scenes at Old Trafford.
The transfer window opens in 15 days time and Mourinho is keen to shake up his misfiring squad.
But he is growing concerned that United’s board appear reluctant to back him in the transfer market by lining up his targets early.
It could be the strongest sign yet that their executive vice chairman Ed Woodward is considering sacking Mourinho unless results improve and is not prepared to give him more funds to spend on signings.
A source close to Mourinho said: “We are near the beginning of the transfer window and nobody knows anything.
“They are not working on selling players, they are not workings on buying players and they have 58 scouts and three scouting bosses. It is ridiculous.
“Jose believes this is not the best way to rule a football club.”
United lost to Valencia in the Champions League on Wednesday to finish runners up in their group and are in sixth place in the Premier League, 16 points behind leaders Liverpool, who they play on Sunday.