However, Chelsea cut short the deal in January, stating they wanted to integrate Casadei into the first team.
However, he has barely played, featuring in 17 minutes for the Blues first team this term.
Before an FA Cup loss to the Blues on Sunday, Maresca stated: "I don't know. I know that Cesare was happy here. We were happy with Cesare.
"The reason that they decided to bring him back, I don't know. The only thing that I can say is that we lost players in January. The reason why is a question for Chelsea."
We started with Wilf (Ndidi) then he was out for more than two months. Then we lost Cesare in the moment that we lost Wilf," he continued.
"We tried to use Ricardo (Pereira), Dennis (Praet) and Yunus (Akgun) with Kiernan (Dewsbury-Hall) but it's not exactly the same. Even with Cesare, we are still there (top of the table in the Championship)."