Aguero is back in training after four games out with a hamstring injury but will not be considered for Saturday's visit of Southampton due to a lack of match fitness.
That makes the following week's trip to Anfield, for what is set to be a huge fixture in the race for the Barclays Premier League crown, the comeback target.
Pellegrini said: "We hope, with one week more working, he will be perfect to play against Liverpool."
Pellegrini had hoped Aguero would be back in contention to face Southampton at the Etihad Stadium, but the player himself this week suggested that would be unlikely.
The Argentina international, 25, was quoted saying he was mindful of rushing back too soon and jeopardising his chances of featuring in later matches, or even the World Cup.
Aguero has scored 26 goals for City this season but none since January due to hamstring problems.
Pellegrini said: "Sergio is very important for us in every game.
"It is a pity he can't arrive 100 per cent now for Southampton because, I repeat, we are not thinking about Liverpool (yet), we are thinking about Southampton.
"But of course he will be very useful for us because he is a top player and he makes a difference.
"Sergio has been very unlucky this season with his injuries, but I hope from now until the end of the season he will play regularly without any problem.
"It's very important also for him to play before the World Cup so that he arrives in the competition at his best moment."
Pellegrini is aware of the big focus already on the Liverpool game, but is keen to put that aside for now.
He said: "We are not thinking about the title, not thinking about the next game against Liverpool. Before that, we have to win against Southampton.
"After Southampton, we play with Liverpool - but then that is not the final of the Premier League.
"The other teams will have 12 points to play for and we will have 18. So there is a lot of work to do after that game."