McTominay with Pogba next to him, is different from Herrera with Carrick/Blind behind him. McTominay is absolutely there to protect the house so the speak.
You may think that McTominay is better suited to being an 8 than a 6, but that doesn't change the fact that he plays much more reserved for us than Herrera did when he scored his goals.
LVG almost always plays with 1 holding MF, 1 box-to-box, 1 no 10. Under LVG, Herrera was always an 8. With Carrick or Blind behind him.
Herrera had 8 goals, 5 assists in his first season. 5G 5A in his 2nd season and when he started player deeper he wasn't scoring as often, because he wasn't at the edge of the box or in the box as often. Some assists in his 3rd and then his goal contributions dropped off.
McTominay is very careful with choosing his moments to go forward. Remember the EL final or the extra time shot against West Ham? He doesn't get in those positions often from open play. McTominay is a worse passer than Herrera, that's why he has less assists, but the difference in goals is a result of different tactics.
When McTominay scored that goal against West Ham he entered the game in the 73rd and was expected to add something to the attack. Fred sat back and McTominay came on for Matic. For me both McTominay and Herrera play(ed) similar roles for united for similarly long periods.
Herrera was better at balancing his defensive and attacking responsibilities as a number 8. Under Mourinho I'll agree he was less of a number 8 and expected to be more defensive, less explosive and mobile.
That West Ham goal is very much a standard type of goal McTominay scores (ghosting into the box, a header off a set piece or edge of the box goal are his types). If he was a better number 8 he'd be getting into those positions more often. He's given freedom to
McTominay is never entrusted with playing as the deepest midfielder. Maybe you can point to a few examples where he actually was deployed as such from the start? Fred is always deeper, and he won 2.7 tackles per 90 in the PL last season to McTominay's 1.6.
Fred also had 1.5 interceptions to McTominay's 0.9 in the same competition. Passing wise Fred attempted 60.2 a game to McTominay's 40.6. How is a player suited to number 6 constantly attempting the least passes out of midfield? It's almost impossible actually, unless the player is hiding. Fred takes those deep lying responsibilities more often, and he's seemingly preferred to do it over McTominay.
McTominay is expected to be the box to box player with possession and a basic disrupter off it when helping with the press.
He's just not very good at being a number 8 and thrives in the simple role of harassing opposition players in possession. This is why his best performances come against big clubs that keep possession because he can focus more on that role
It's not a simple matter of him 'carefully' picking when to go forward, he's expected to (his goals last season from open play were similar to the West Ham goal), it's merely his poor positioning, poor reading of the game, not picking the right times to make forward runs/join the attack and reluctance to make himself available for the ball that lets him down. I don't think he has absolute confidence in his own on-the-ball and passing abilities. It's one reason why so many in matchday threads often mention him going missing in certain games
He will try and compensate with energetically running like a headless chicken a few times per game and it will be received with cheers and applause here despite those runs seldom ever being productive, like the EL final. All workrate and energy, zero game changing quality.
He mostly clatters into players, wins fouls here or there, but hardly ever provides genuine quality going forward despite being given the freedom to do so when the moment fits. He's a poor number 8, never a 6