Pellegrini's side have started the campaign unbeaten - keeping two clean sheets - with Vincent Kompany and Eliaquim Mangala the central defence pairing which has seen the captain score in both games.
Martin Demichelis has been forced to take a seat on the bench and his fellow Argentine team-mate, Otamendi, may be joining him for the time being as he fights for his place.
"He [Otamendi] is a very good player who is improving," Pellegrini told reporters. "Last season with Valencia he made a very good season so he is improving every year, and he's a good header [of the ball] which I think is a good thing in England.
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Manuel Pellegrini says Nicolas Otamendi will have to work hard for his place in his side
"[But] the way we are playing it's impossible to move Kompany and Mangala out of their positions. We are in a good moment, we haven't conceded a goal so we will continue to play with the same defence."
Manchester City travel to Goodison Park on Sunday hoping to stretch their lead at the top of the table while Everton are still searching for their first win of the season.
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