Erik ten Hag thought his side took an early lead on Saturday when McTominay tucked in Alejandro Garnacho's pass across goal from a free-kick.
But as VAR intervened, it spotted that Harry Maguire had made an effort towards the ball and was in an offside position.
Referee John Brooks was referred to the pitchside monitor and after nearly four minutes, the decision was made to rule the goal out.
It left United fans red with rage, including this one who said: "Maguire isn't even off the defenders arm plays him on plus it's debatable if he's in play when he absolutely misses the ball… Absolute joke.
"Talking of which… How many electricians does it take to change a light bulb comes to mind."
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While this fan said: "When will real change in relation to VAR actually happen? It's baffling."
A third put: "Offside now, even if you're not involved in the play. That's new. Never been a thing before but now it's United playing.
"Let's get a replay or something from when Bruno was downed in the box. City got one against us for less!"
United are in desperate need of a victory with pressure ramping up on Ten Hag.
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Two heavy defeats in a row to Newcastle and Manchester City have seen reports emerge that the Dutchman is fighting for his job.
talkSPORT understands that while Ten Hag's position is not under immediate threat, results will need to improve drastically soon.
It was no wonder, then, to see the United coach venting his anger at the decision to rule out McTominay's goal.
Especially considering that City had a goal allowed despite a similar situation earlier this season.
Nevertheless, the Red Devils have looked lacklustre against Fulham in a match lacking much quality.
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