The Argentinian forward, who has already netted 26 goals in his 25 appearances this term, has spent the last month sidelined with a hamstring injury.
Aguero's absence has resulted in a lean patch for City, who have managed to score just three goals in five games without their leading striker.
The 25-year-old is set to return to full training on Tuesday and with a Capital One Cup final date with Sunderland at Wembley on Sunday, Pellegrini admits Aguero will be a welcome addition to his ranks.
"Sergio can give us all a huge lift, not just for the Capital One Cup final but for the rest of the season because, for me, he is one of the top five players in the world," the City head coach told The Sun.
"He has had two long injuries but when he's fit, he is a player who can make the difference."
There could be a double boost for Pellegrini ahead of Sunday's final, with Serbian defender Matija Nastasic also on the road to recovery following a knee injury.