Bellingham proved the matchwinner in England's Euros victory against Serbia on Sunday.
Asked if the Real Madrid midfielder could become the best in the world, Mainoo said: "Yeah definitely. I mean everyone knows how great a player he is. We see it everyday in training.
"He's such a nice guy as well. He's demanding, he has high standards that he sets and to play with him is great.
"Dec (Rice) is also such a good player, so reliable and so good on the ball and dominant in midfield, so when I did get to play with him it's always a joy to have someone so strong on the ball next to you. You can always trust him no matter where you are on the pitch.
"Everyone knows he's worth every penny, he's such a good player. He's proved that in the Premier League and also on the international level.
")Playing alongside Rice), it frees me up a lot and allows me to go forward and attack and I always know that when I go I can trust that he's there. It's a dream."