The Blues initially considered loaning Casadei to gain more experience.
However, the 21-year-old former Inter Milan midfielder wants to leave permanently, because he will not get playing time at Stamford Bridge.
According to Gianluca Di Marzio, Casadei has agreed to join Torino in Italy.
The two clubs are now working on a deal, which could be worth around £10 million.
Chelsea are willing to lose the player for a modest fee in exchange for taking up a buyback option.