After playing for the Rams on loan from West Bromwich Albion last season, 21-year-old Thorne made the switch to the iPro Stadium permanent over the summer but then ruptured his anterior cruciate ligament in a pre-season friendly.
"Everyone knows you can't rush these injuries and I'll be back when I am ready and it feels right," Thorne, who was ruled out for nine months, told the club website.
"That may be sooner, but it could be later – it really depends how things pan out.
"I've had a successful operation and a good few weeks of rehabilitation but I have to take it each day and week as it comes."
The central midfielder put pen to paper on a four-year contract with the Championship club.