McTominay was unable to influence his side's opening Premier League game at Old Trafford on Sunday.
He couldn't get going in midfield as the Red Devils suffered a shock 2-1 home defeat to Graham Potter's team.
The 25-year-old was subjected to intense criticism by fans online following the result.
His lacklustre performance has since been compiled in a video and shared on social media.
Former Man United greats Roy Keane and Paul Scholes didn't hold back as they slammed McTominay's showing against Brighton.
Speaking after the game, Keane said: "I've been going on about it for a long time - Fred and [Scott] McTominay are not good enough.
"They won't get Manchester United back competing at the top. We see it week in, week out. They're not up to it."
Scholes added: "McTominay was always trying to run with the ball, ridiculous. The quality in the middle of the pitch was really poor.
"I was happy he didn't get sent off [for a high challenge] but I think he was slightly lucky. Again, he's running with the ball, absolutely ridiculous.
"Fred gives the ball away all the time and McTominay's trying to run with the ball constantly and then gives it away. Pass it to your best players! It's not that difficult. It's criminal on a day like today."
McTominay played 30 league games last season and started Erik ten Hag's first competitive match for Man United.
He was eventually dragged off in the 78th minute for Donny van de Beek and his spot in the team could be under threat with the club linked with a move for Adrien Rabiot.
It comes after Man United have yet to secure the signing of Frenkie de Jong, who remains Ten Hag's No.1 transfer target.
Featured Image Credit: Premier League
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