The Rams are trying to cope with a crippling injury list.
Around a dozen players are sidelined and only a handful of them could be available for the trip to midlands rivals Leicester when the Championship programme resumes on October 17 after the international break.
Green recently underwent a second operation on the foot he broke in March. He had a longer screw inserted into the fracture and it was hoped the 26-year-old could be fit before the end of the month.
However, manager Nigel Clough said: "Greenie has seen the consultant again and he's not 100% happy to let him start running in the next week or two.
"So he's going to be at least four weeks, which is the biggest blow."