Liverpool have won both of their first two league fixtures under new manager Arne Slot but face their biggest test so far against United.
United go into the game after dropping points against Brighton last weekend and will be looking to avenge that performance.
Mainoo spoke about the importance of the game and how the crowd will help in securing three points.
“For both of us, it’s a huge game so early in the season.
“I feel like it will have a big impact on how the start of the season will map out, because it's such a big fixture, it will affect us.
“So yeah, we'll have to prepare right and we'll be ready for it.”
The young midfielder wants fans to be the 12th man on Sunday as a great atmosphere can make all the difference.
“Both games at Old Trafford, it was rocking. It was amazing. So that definitely helps us.
“It’s like a 12th man on the pitch for us when it's like that, so we appreciate that.”