Aguero continues to be the club's lucky charm - City have never lost when he scored, and this season he has scored nine in nine starts.
Their Premier League title credentials will be put to the test against Chelsea at Stamford Bridge on Sunday - but Pellegrini believes with Aguero continuing to develop his game they can be confident.
"He is a top player. I always talk with Sergio about the fact that he, for me, is one of the five top players in the world," said the City boss.
"He must try to demonstrate it in every game he plays. He is playing with more intensity when we don't have the ball and he has a lot of quality to make the difference.
"I didn't know the statistics which said City have never lost when he scored but he is an important player for us.
"He was an important player also in the past - with his goal Manchester City won the title. We hope in the future he will continue being a very important player for the club."
Sunday's game is one City cannot really afford to lose as it would open up a four-point gap with one of their title rivals.
"It is a tough game for Manchester City against Chelsea," he added. "We are one point behind them and must play a very good match if we want to win."