On Saturday Nasri comes up against former club Arsenal as the two sides meet in the Premier League's lunchtime kick-off at the London Stadium.
"I think they can expect a lot from him because he is a top player," said Pellegrini, who managed Nasri for three years at Manchester City.
"The way he continues having some minutes, when he is used to playing 90 minutes again, he will be a top player at this club.
"He wants revenge also and for me it is so important to give him the opportunity to work with us so I hope we will recover the same Samir Nasri that we saw three or four years ago with me at Manchester City.
"[He wants revenge for] the ban of twelve months or more, to decline his career. From Arsenal, City bought him. From there his career declined without doubt."