The side have never lost when the Argentina international has scored and nine goals in as many starts this season underline how important he is to their future success.
Their championship 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.
"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.
Pellegrini has, in his own way, described it as a "six-pointer".
"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."
While Aguero is leading the line so well up front midfielder Samir Nasri believes the partnerships working behind the striker could become increasingly significant.
The France international thinks his form and that of Spain's David Silva could provide an important edge to their title challenge with Nasri already having four assists to his name and Silva five.
"When I first arrived here, we played a lot together and we won the Premier League," Nasri told mcfc.co.uk.
"We play a lot of possession football so it's better to have two playmakers on the pitch to help keep the ball and can make something happen with a pass or with a piece of skill.
"The style we are playing suits my game and it suits David's because when you have the ball, there is always a solution and that's how it needs to be if you want to play attractive football.
"We play one-touch, two-touch or whatever is needed to move the ball forward and at pace higher up the pitch.
"It's a good mentality, it's what the manager and we have to keep it up.
"I especially love the big games because those are the ones everybody wants to play in and it's true I enjoy playing against Chelsea."
Captain Vincent Kompany will be absent again at Stamford Bridge as he has not yet recovered from a thigh injury sustained against Everton earlier this month.
"Vincent is still injured, so he will not have a chance to play this game," Pellegrini said.
"Jack Rodwell is okay and all the others are fit, we have no more injuries."
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