Giles Barnes has undergone a knee operation in the United States and faces a lengthy spell on the sidelines.
"We are waiting to hear how the operation has gone," said Rams boss Paul Jewell after Wednesday night's 6-1 defeat at Chelsea.
Asked how long he will be out, Jewell added: "It is going to be some time."
Barnes, who suffered the knee injury in training, has already been ruled out for the season.
Jewell suggested the 19-year-old attacking midfielder could need further surgery on what is believed to be a problem with his kneecap.
