Pellegrini appears to have some selection issues looming as Aguero gets back into his stride after a month out injured.
Aguero made a goalscoring return off the bench after an eight-game lay-off against Blackburn in the FA Cup midweek.
His strike - just 50 seconds after coming on - was his 20th of a prolific campaign but it says much for City's fearsome attack that he was hardly missed during his absence.
Alvaro Negredo and Edin Dzeko, who have scored 21 and 15 goals respectively, led the line well during the Argentinian's time out.
Yet although Aguero is again set to be among the substitutes again against Cardiff this weekend, the 25-year-old is soon likely to be back starting.
Pellegrini said: "It would be difficult to leave him out.
"We've got so many games to play that we need the best performances from them.
"We'll see which is the best way to start the next one.
"For the moment Sergio is just coming back from an injury and he's not fit yet to play 90 minutes."
City's 5-0 win over Blackburn, which secured a fourth-round tie with Watford, was the latest in a series of big wins at the Etihad Stadium.
It also took their goal tally for the season to 99 and, as they average four a game at home, the century seems likely to be broken as Cardiff visit in the Barclays Premier League on Saturday.
For now Negredo is their most potent goalscoring weapon with the Spaniard having scored seven in his last four games.
The 28-year-old has been an instant hit since his move from Sevilla last summer, which at £20million came at a fraction of the cost paid for Edinson Cavani and Radamel Falcao elsewhere in Europe.
Pellegrini, a former Malaga, Real Madrid and Villarreal coach, said: "All of them are very important strikers, top players.
"But I thought that here we had Dzeko, so if we could bring in Negredo as well we could be very strong with both of them.
"I think both of them are playing the way they can. We are very happy with them and now we don't need any more strikers.
"I thought that at the beginning of the season too. They are top players."
While the goals have hardly stopped flowing this season, City have occasionally shown vulnerability at the back.