Toro president Urbano Cairo triggered the permanent option in the defender's loan deal last season.
Aina told Torino Channel: "I have to improve in the final phase of the action, and I am convinced that we can succeed.
"I am working hard to be ready when I'm needed. Ours is a very strong squad, it will be difficult to find space but I will do my best to succeed, thank you to the club for having exercised the option with Chelsea.
"Now we are in Europe and we have deserved it, because to get there we have committed so much and now we want to enter the group phase."