The defender joined the Premier League champions from Fiorentina on deadline day in a deal worth around £12m.
"I think Matija is going to be a top player," said Kolarov. "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."
Kolarov and Nastasic both played for Serbia in Tuesday's 6-1 World Cup qualifying win over Wales in Cardiff.