City enhanced their Premier League title credentials and inflicted a 'double' on their arch-rivals by easing to a 3-0 victory at Old Trafford on Tuesday night.
Pellegrini echoed two-goal hero Edin Dzeko's comments that the win was crucial, and was delighted to emerge from the possible distraction of his second Manchester derby unscathed and in such style.
"It was very important psychologically because it was one of the three big matches we had still to play, and it was against a great team like Manchester United," he said.
"We continue recovering points on Chelsea, Liverpool and Arsenal so it was very important to keep adding points.
"We need to win more points than the other teams but you'll never know how many points you'll need over the other teams."
It was a fifth straight clean sheet for City and, while revelling in another goal glut, Pellegrini is enjoying the current cohesion between defence and attack.
He added: "For me, it's very important to keep a clean sheet. It's important to score goals but today we worked very compactly as a team.
"We didn't give them a metre to make moves in our defence and so we were good in both attack and defence.
"I think we are a very balanced team at the moment."
The result puts City within three points of leaders Chelsea with two matches in hand, but at his post-match press conference, Pellegrini refused to entertain talk of the title being theirs to lose.
"I don't think so," said the Chilean. "We will continue fighting with the other three teams, Arsenal, Chelsea and Liverpool, and we have to win the two postponed games and continue playing the way we play.
"I am not thinking about the title. Tomorrow we are thinking about Arsenal (whom they face on Saturday)."
In a relatively incident-free Manchester derby, the only controversial moment came in the first half when United midfielder Marouane Fellaini escaped with a yellow card after appearing to catch Pablo Zabaleta in the face with an elbow.
Pellegrini said he saw it but accepted referee Michael Oliver's judgement. United boss David Moyes said he initially did not think it was a foul and had to be told afterwards it was a potential red card.
"I didn't see the incident. If the referee decided not to send him off I respect that," said the City boss.
Moyes added: "I genuinely have not seen it again. When I first saw it in the game, I didn't even think it was a free-kick so I couldn't even tell you, but by what I am hearing, people are saying it could have been worse."