Aguero was missing again for their draw at Norwich City.
Pellegrini said: "It is difficult coping without the spark of Aguero. Because we have been scoring goals so easily this season people have been talking about how many more there will be but scoring is always the most difficult thing in football.
"I think the problem in the last two games is absolutely related to Aguero's injury and Negredo and Dzeko being not fit enough to play as well as they did before. We are in an unlucky moment of the season but we are right behind the leaders and have 39 more points to fight for.
"This team were favourites to win the title with the way we were playing. We must try as soon as possible to play that way again."