McTominay was again left out of the starting side for the European contest, but he made a telling contribution late on, firing the home team's winner in second-half stoppage time at Old Trafford.
Man United remain second in Group E, but they are just three points behind leaders Real Sociedad and crucially still in with a chance of finishing at the head of the section.
After the match, McTominay said that the importance of topping the group, and securing an automatic spot in the knockout round of the competition, had been stressed to the players by the coaching staff.
"That's massive for us," McTominay told BT Sport. "That was the message before the game, strongly reiterated by the coaches - win the group. Everyone wants to win the group, it's important for the games coming ahead so, as I say, for us, it's massive."
McTominay also paid tribute to Omonia goalkeeper Francis Uzoho, who made 12 saves during an incredible display at Old Trafford.
"He was outstanding and sometimes you think it's not going to be your day but, as I say, credit to the team," the Scotland international added.
"You have to be patient. Marcus [Rashford] had so many opportunities and he's come in at the end saying 'I could have been there all day and the ball wouldn't have gone in the net'.
"He's done pretty much everything but score, but yeah as I say - we hit the crossbar with Casemiro. Their goalkeeper, I've just seen him then, was different class, to be fair to him, so fair play to them."
McTominay also said that the "team spirit within the group" played an important role in the home side eventually making the breakthrough.
"We've got a really good team spirit within the group, where everyone keeps going and fighting for your place every single week and, for us, their goalie was outstanding which is unfortunate but we managed to get the goal in the end, which was good," he added.
"They're sat in deeper and deeper as the game goes on so there's a lot of space towards the edge of the box and, obviously, that was the place where we could exploit that - obviously with shots at the edge of the box.
"It's important that we dial into every game and obviously it's a squad game, like you see with the lads who have come off the bench against Everton, the game before that obviously and tonight. It's what the manager wants - so yeah everyone's pleased."
Man United are now preparing for two home Premier League matches in quick succession, with Newcastle United visiting Old Trafford on Sunday before Tottenham Hotspur head to Manchester next Wednesday.