Louis van Gaal faces the sack as United manager if his side lose either of their next two Premier League matches.
Van Gaal is under increasing pressure after United suffered a third consecutive defeat and slipped out of the top four with defeat at home to newly promoted Norwich on Saturday.
The Glazer family, the club's owners, and executive vice-chairman Ed Woodward have been increasingly uncomfortable with the level of unrest among the club's support.
It is understood there is also discontent within the squad regarding tactics, system and his coaching methods.
The Reds travel to Stoke on Boxing Day- always a difficult proposition- before hosting champions Chelsea two days later.
There is concern among the United hierarchy that failure in these two matches will leave the club in a perilous position as we attempt to get back on track in the Premier League title race.
Jose Mourinho is understood to be interested in taking charge at Old Trafford having been sacked by Chelsea while Pep Guardiola could also be considered as a contender having yesterday announced that he will be leaving Bayern at the end of the season.
Van Gaal admitted in the immediate aftermath of Saturday's loss that he was concerned for his job, and has stated that he will resign if he feels he has lost the support of the players.
But asked if that was the case, he said:
"No because I have seen the good reaction of the players towards me"
He went on to suggest that he still feels he has the board's support having spoken to Woodward at the weekend.
Paul Scholes, who won two Champions Leagues and 11 league titles under Sir Alex, has been among the fierce critics of Van Gaal, saying he would not have enjoyed playing in this side under the Dutchman.
Former United captain Rio Ferdinand believes they should move for Guardiola- who led Barcelona to the Champions League title twice and has won the German title in each of his two full seasons at Bayern Munich.
The fact that Mourinho is now a free agent and is believed to be keen to make a quick return to management makes him a more immediate threat to Van Gaal.
The Dutchman and his side did little to calm recent disquiet as they lost to Norwich, managing just two shots on target.
Van Gaal has admitted United are in a "bad period" and that he believes he will be given time to turn recent form around.