The Blues offered the 18-year-old a five-year contract after his impressive performances in pre-season, but he is holding off on putting his signature to it.
“He has to improve. He has to improve a lot," Sarri said at a press conference ahead of his side's game against PAOK in the Europa League on Thursday. "First of all, in the defensive phase, he has to work. He has to improve in the offensive phase – he's very good one against one, but he has to learn to move without the ball.
"He is potentially a very good player, and now he has to study and improve and work. But I think that's really very normal. He's 18, so that's normal."
With Eden Hazard and Victor Moses missing the midweek clash at Stamford Bridge, Sarri said he will try to include him in the squad, but remained non-committal.
"I want to find space for him. It's not really easy because, at the moment, we have 28 players," he added. "It's not easy to put him in the starting XI because, maybe, on the bench in the Premier League matches there was Victor Moses or Pedro, so it's difficult not to put that kind of player in the starting XI for Thursday.
"So it's difficult, but I'll try to find a space for him. He is in the 18 players for the match. I don't know if he will start or not.”