The club were updating their live blog during the emphatic 2-2 draw with Southampton when it contained the explicit tweet.
The tweet itself was praising striker Marcus Rashford's performance after the England striker assisted two to salvage a point.
But it was the supporter's Twitter name that left the club red-faced.
Jose Mourinho is under massive pressure as United boss after his side dropped eight points off the top four.
They have only picked up six wins in 14 games and unbelievably have a negative goal difference.
According to the Times, the Twitter user could have seen his tweet on the live blog and then changed the name of his handle to reflect his views.
Mourinho risked further wrath from the United dressing room after accusing £89million midfielder Paul Pogba of being a virus who is infecting his team.
He launched the astonishing assault in-front of his own team-mates following the draw.
An Old Trafford source revealed “Jose called Pogba a virus in front of all the players.
“He told him ‘you are like a person with a flu with a virus in a closed room – you pass that virus to the others.'
“He also told him ‘you don’t play, you don't respect players and supporters and you kill the mentality of good, honest people around you.'”
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