The Argentina international was at his brilliant best at the Stadium of Light on Wednesday, scoring two goals and setting up another as City came from behind to beat Sunderland 4-1 in the Premier League.
His double, and goals from Stevan Jovetic and Pablo Zabaleta, ensured the champions remain six points behind leaders Chelsea and are now four better off than third-placed Southampton.
Aguero now has 19 goals in all competitions this season and Pellegrini believes there is plenty more to come from the former Atletico Madrid man.
He said: "I am very happy for Kun because he is working hard. I always say he is too young. He is only 26 years old and I think he can improve every day.
"He wants to do it and at this moment, it is difficult for defences to stop him. He is playing a very high performance but there are so many years left for him to strengthen.
"Before he started scoring in these games in a row, I thought he was in the top five players in the world. I am very happy for him because he's playing well as well as scoring."
Pellegrini is confident City, who impressively beat Southampton 3-0 on Sunday, are heading in the right direction.
The Chilean added: "This team is not just about Aguero though.
"He is a very good player but the team have come here again and played the same way we did last season - we dominated the game and we have players in high performance.
"We are playing very well and we repeated the same performance we did against Southampton - we were very strong.
"It's very important to be an attractive, offensive team for the fans."
