The youngster was hoping to get more game time under boss Enzo Maresca, who managed him at Leicester City.
However, Casadei is on the periphery of a very strong and deep Blues first team squad at present.
“When I saw Maresca arrive at Stamford Bridge in the summer I was excited," he explained to Italian publication Tuttosport.
"We already knew each other because he was my coach in the first six months of last season at Leicester City.”
On the transfer speculation, he added: “The transfer rumours about Napoli, Milan and even Juve? After speaking to the coach, I was 100% sure that the decision to stay would be the best.
"Maresca told me what he thought, I had no doubts. I stayed here without considering possible loans. Nor do I think so at the moment for the next winter transfer market.
"I couldn’t be more proud to play for Chelsea. The competitive spirit was one of the reasons why I decided not to leave."