City take on the Londoners in the FA Cup fifth round this weekend having already lost to Jose Mourinho's men in both their Barclays Premier League meetings this season.
A defensive mix-up between Joe Hart and Matija Nastasic was blamed for the first loss at Stamford Bridge but fingers were pointed at Pellegrini after the reverse fixture at the Etihad Stadium last week.
Pellegrini was widely viewed to have got it tactically wrong after the 1-0 loss having stuck to his default 4-4-2 formation, seen his side overrun in midfield and picked off on the counter-attack.
Yet the Chilean has not veered from his basic attacking philosophies all season and he is not changing now.
He said: "I am absolutely sure that in the previous game it was not a problem with tactics.
"The squad had a lot of changes and we had three or four clear chances before they scored.
"If we'd scored we could have scored again.
"The goal they got, he (Branislav Ivanovic) does not find it easy to shoot with his left foot - but he did it in that game.
"If we'd scored first the result could have been different but the tactics would have been the same."
City were weakened in midfield by the absence of the influential Fernandinho for that last encounter, and the Brazilian remains on the sidelines with a muscle strain.
Defender Martin Demichelis made an unimpressive appearance in midfield in his place but Pellegrini insists this did not necessarily weaken the team.
He said: "They had a shot from (Nemanja) Matic and a header against the post from (Gary) Cahill - but they didn't have any more chances in the second half.
"In the first half they had one when we missed a pass and it resulted in a counter-attack, and they scored the goal they got.
"It was not because we didn't have Fernandinho, it was because we made a mistake with a pass. We were not in order."
City have playmaker Samir Nasri available again after five weeks out with a knee injury and the France international is set for some action, with Tuesday's Champions League match against Barcelona in mind.
Pellegrini said: "I think one of the most important things in a recovery is to play a bit. He is not ready to play 90 minutes but it will be useful for him to play some minutes."