However, Casemiro believes that more game time and experience will bring out much more in Mainoo.
"First off, I'd like to congratulate Kobbie. He's only 18 and he's already a starter at a club like Manchester United," said Casemiro, in an upcoming episode of United's TeamViewer Diaries.
"It's not easy, there's a lot of pressure. But I think United will have a top player for the next years, easily. He's a top player. He's exceptional. He's a humble, hard-working player. He knows how to listen. He's a great player.
"Since I play alongside him, I know Manchester United will have one of the best midfielders in the Premier League for the next years, easily.
"I think we have to recognise his value. He's a great kid. I'm very happy that he's on our side. As long as he keeps his head on straight, just as he's doing now, I think we'll have a great player for many, many years here at United."