And despite Aguero's stunning form for City this season, manager Pellegrini is convinced the Argentinian is going to get even better in the coming years.
Aguero continued his outstanding start to the season with a sensational hat-trick in City's dramatic 3-2 defeat of Bayern Munich in the Champions League in midweek.
The prolific Argentinian has now scored 17 goals this season and is again being compared with Barcelona star Messi and Real Madrid ace Ronaldo, the pair long acknowledged as the best in the game.
Pellegrini said: "Well of course I think that no-one can replace Messi at Barcelona, Ronaldo at Real Madrid or Sergio here - or a lot of very important players
It is not the same playing without Aguero as with Aguero.
"He has many years more to play here
I am always asking and demanding that he tries to be the best player because he has all the conditions to do that.
"That is why I am very glad for him he is always improving every day.
"I think he is a top player, a very important striker here in the Premier League and we are very glad to have him in our team."
Aguero, 26, is set to make his 100th Barclays Premier League appearance since joining City for £38million in 2011 as the champions travel to Southampton this weekend.
Only Alan Shearer (79) and Ruud van Nistelrooy (68) have scored more goals in their first 100 appearances than Aguero has in his first 99 (64).
Despite his prolific output, and virtually beating Bayern single-handedly in midweek, Pellegrini insists City are not a one-man team.
Aguero did score 28 goals in all competitions last season but his contribution in the second half of the campaign, as City closed out title success, was limited due to injuries.
Pellegrini said: "I always say we have a squad and if, for different reasons, Sergio cannot play, we must have another one that can do it.
"We hope he continues the same way but I suppose Edin (Dzeko) will be fit next week and he can score also
This team we never depend just on one striker
The last (league) game, Sergio didn't score and we won with a goal from (Stevan) Jovetic and one from Yaya Toure
If you remember, we won the title here last year with a lot of games without Sergio.
"But, when he plays, it makes a difference."
Aguero was injured in January and in and out of the team for the rest of the season after that, and also hampered at the World Cup.
Because of that Pellegrini eased him back into action at the start of this campaign, and that decision looks to have paid off.
Pellegrini said: "You never know when a player will be injured but I think that this year we prepared him in the correct way.
"If you remember in the first three games, including the Community Shield, we didn't play with Sergio, we just gave him 20-25 minutes, no more, so he could have a good pre-season
He needed that after the last season when he had a lot of injuries.
"He is fit and we hope he will continue in this way until the end of the season."
Aguero has said that he views the game at Southampton as must-win, as it would enable City to leapfrog the high-flying Saints into second place.
City beat Swansea last weekend and are hoping to build some momentum with further games against Sunderland and Everton to come in the next week.
Pellegrini said: "Yes, it is an important game because we need the three points and we can be in the second place
It is always better to be second than third but it is not only this game.
"This is the first game of the three we must play this week in the Premier League and I hope after the three we can be second place and be near to Chelsea."
Source : PA
Source: PA