The 18 year-old has tasted first team football this season.
Vieira told the Manchester Evening News: "Marcos is a fantastic talent. He shows it week after week at youth level, but when you go to the first team it is a different level. He played really well at West Ham, and showed he can be a player for City in the next few years.
"What is important is he showed how good he is but at the same time he knows he has not finished improving and learning.
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