Bolstering a squad is more than signing a new forward, the defense must be up-to-date as well and City have done that with Otamendi joining for the next five years.
The Argentina defender was once targeted by United, but is moving to the sky blue part of Manchester from Valencia for a fee believed to be worth £31.7 million.
The 27-year-old will compete with Mangala for a starting position after signing a five-year contract.
Otamendi is ambitious enough to already target winning trophies with city, who have rumbled to the start of the season.
“I am here to give my best, to fight game by game and push City to the highest peaks for as long as possible, and I wish we will be able to win several cups and titles,” Otamendi told the club’s official website.
“That’s the most important [thing]. To be here inside a club that looks so wonderful from the outside is a dream.”
Pellegrini added: “Nicolas Otamendi was arguably the best defender in La Liga last season and so naturally I’m delighted to have added a player of his quality.
“He is an established international for one of the best nations in world football and I have no doubt that he has the right mentality to fit into a squad where he will find many familiar faces, including Eliaquim and Fernando who he played alongside in Portugal.
“As a footballer, Nicolas has all of the qualities to be a success in the Premier League – he’s strong, he’s excellent in the tackle and he’s very good technically.
“I am sure he will become another Argentine player our fans will enjoy watching.”