The 22-year-old became Antonio Conte's first signing in West London after completing his £33million move from Marseille on a five-year contract.
In his first interview with the club's official website, he said: "It feels fantastic to be here, it's a childhood dream for me.
"It's wonderful. It's very big and you feel that you've joined a club that really want to win."
When asked whether he was a fan of Premier League, he added: "Yes, everybody watches Premier League football and of course I've seen a lot of Chelsea matches."