The 60-year-old Chilean coach, in his first season at the Etihad Stadium, delivered the line straight-faced in reply to a question at a news conference when he was asked if a City victory over Barcelona would change the perception that United were the biggest club in Manchester.
"If we consider only this season there is just one club in Manchester and it is ours," he said.
"Basically the biggest club is now City, though we cannot forget the last few years."
Pellegrini, not normally as prone to barbed one-liners as managers including Chelsea's Jose Mourinho, may be factually correct regarding the form of United and City this season.
However, United's achievements, which include winning the European Cup three times and 20 English league titles, totally dwarf those of City who have been English champions three times and are in the knockout round of the Champions League for the first time.
Pellegrini did say that even if City do eventually knock out Barcelona, they have some way to go before they can be considered among Europe's elite as they could easily be knocked out in the next round of the competition.
City were dismissed as "noisy neighbours" in 2009 by then-United manager Alex Ferguson when they began to flex their new-found wealth following the investment in the club by Sheikh Mansour of Abu Dhabi.
But they have certainly made more of the right noises than United since Ferguson's departure at the end of last season.
They are currently third in the Premier League, three points behind leaders Chelsea with a match in hand, and have reached the Capital One Cup final and the FA Cup quarter-finals.
In contrast, United are seventh in the Premier League, 15 points behind Chelsea, and although they are still in the Champions League and face Olympiakos of Greece in the round of 16, they are out of the FA Cup and lost in the League Cup semis.
However, they have their own financial muscle, glamour and history to attract the best players in the world and come back strongly next season.
In the meantime, Pellegrini is enjoying the moment.
"Of course our supporters are happy this season," he said.
"We are still in four competitions, they enjoy watching the way we play and the goals we are scoring and we want to win every competition we are in. Of course, I understand where this club were 15 years ago, in the third division, and where it is now.
"But we can beat Barcelona in this round and go out in the next round and we have not reached all of our objectives yet."
Pellegrini also accepted that Barcelona pose the biggest threat yet to City's ambitions of establishing themselves amongst the elite.
"They are not just a one-man team called Lionel Messi," he said.
"They are a different team to the one that won the Champions League three years ago, sure, but they still have great players.
"We can only play them one way, the way we have played all season and cannot change our style to face them, although you have to consider Messi when you play against Barcelona.
"They are top of the Spanish League, in the Spanish Cup final and are still one of the most important teams in the world with the best player in the world.
"So it should be quite some game here tomorrow."
City, who have netted 117 goals in all competitions this season and are the highest scorers in this season's Champions League with 28, will miss dangerous striker Sergio Aguero on Tuesday.
Pellegrini confirmed he had not recovered sufficiently from a hamstring injury he sustained last month and is also doubtful for Saturday's Premier League match against Stoke City.