The former Arsenal and Manchester City midfielder signed for the Hammers on a free transfer this week after completing an 18-month doping ban.
The 31-year-old was an unused substitute in the Hammers 2-2 draw with Brighton on Wednesday but Pellegrini assured that Nasri, who was an integral member of his 2014 title-winning City side, is raring to pick up his form of old.
“He has been training with us for a month, he is looking very well, he lost weight,” said Pellegrini. “And I think that football is like when you ride a bicycle when you are a small boy, you never forget.
“Samir is a very technical player and I am sure he will play in the same way as he did before. I trust in him a lot.
“I suppose because he knows that this is revenge for him, he is motivated, working hard and he has a lot of quality and I am absolutely sure he will be a player that makes a difference.”
When asked about Nasri’s doping offence, however, Pellegrini said that he didn’t want to comment on the past.
“I don’t want to talk about whether it was his fault or not his fault. He was involved,” Pellegrini said.
“I have known Samir Nasri since I was at Manchester City. He was a top player in the first year, one of the reasons we won the title was because of him.
“After speaking to him for a long time about what he was thinking about for the future, I decided to give him an option here at West Ham.
“He worked very hard to be considered and now that’s why he has signed his contract and now he has to demonstrate it on the pitch. But mentally, he is another player with a lot of experience.
“That part of his life, for Nasri it has passed.”
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