Manuel Pellegrini had no doubts about signing Samir Nasri and says he wants to give him the chance to clear his name after an 18-month doping ban.
Nasri breached World Anti-Doping Agency (WADA) rules in 2016 after receiving an intravenous treatment in Los Angeles for illness. An initial six-month ban was extended after appeal.
The 31-year-old has stated that it was because the 'dosage of vitamin' given to him was too high.
He signed a deal until the end of the season with West Ham when his ban expired last month.
On Saturday he comes up against former club Arsenal as the two sides meet in the Premier League's lunchtime kick-off at the London Stadium.
Pellegrini believes Nasri can get back to the level that saw him move from Arsenal to Manchester City for £25million in 2011, and in doing so will get his revenge for the ban, which he feels was unjust and nearly ruined his career.
'I think they can expect a lot from him because he is a top player,' said Pellegrini, who managed Nasri for three years at 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.'
Nasri made his debut for the Hammers in last weekend's FA Cup third round win over Birmingham, playing 59 minutes for the first time in 13 months.
Despite a long time out of the game, Pellegrini feels he was right to give Nasri a chance.
He said: 'Every time you choose a player you have a risk without any doubt. Sometimes when you invest more money, your risk increases.
'When I bring players here I say my responsibility doubles than when players were already here with the squad.
'You must take decisions. This was a risky decision but not a high risk one - because he signed for six months, he didn't want an amount of money to try to win money win in these six months.
'He wanted to demonstrate he was back in football. There's a lot of players you can buy and the risk is high.
'He made no problem with the contract. He was not fighting for more or less money. He had an agreement and he wants to demonstrate between now and June that he is able to be a top player in football.
'I have contact with him, with his agent. I have contact with his agent. I know what happened. I had a long meeting with the player also beforehand we took the decision to make sure everyone is happy when we finish the season.'