The 19-year-old centre-back made a £12million transfer deadline-day move from Fiorentina.
And Kolarov reckons Nastasic will prove to be a smart signing.
He said: "I think Matija is going to be a top player. Already, he is playing for our national team and he is still only young.
"He's only 19 and he can go on to be one of the best defenders around.
"This move to England will be very good for him. City have signed a very good player."