Negredo's treble took him to 18 for the season since arriving in a £20million move from Sevilla last summer.
His form has offset the loss of top scorer Sergio Aguero through injury.
Pellegrini said: "Playing better than the way he did today is not easy.
"He really played a complete game, not only because he scored three goals but because of his link with the midfielders.
"For me, it's not a surprise because I know the way he played in Spain for a lot of years and I think it's also important for Alvaro and Edin Dzeko to play well together, because maybe in other games they have not played as well as they used to."