The Hammers refused to entertain offers for the Frenchman during the transfer window, slapping a prohibitive £100m price tag on the man they bought from Marseille for £10m a year earlier.
“It is key for us if he can produce his best form again," Bilic told West Ham's official website. "He has the qualities that the best players have and can help change a game. They are influencing their team-mates and opponents.
“He makes the team play better and everyone around him is benefiting. The team recognised that last season.
"It is crucial that he stayed. It's crucial that he comes back fit and that he tries to produce what he was doing last year."