Derby's newest recruit George Thorne will undergo surgery after damaging the anterior cruciate ligament in his left knee.
The injury was sustained in the Rams' pre-season loss to Zenit St Petersburg, in Austria, and Thorne was taken off after 34 minutes.
The club originally played down the injury and it was believed he suffered a dead leg and a few knocks to the knee, but it appears to be a lot more serious after he returned to England for a series of scans.
Derby manager Steve McClaren told the club's website: "At the time it looked to be innocuous as there was no-one near George and we thought he had just jarred his knee.
"Scans have revealed that he has ruptured his cruciate ligament and he will be operated on today.
"It is a huge disappointment for George personally and we wish him a speedy recovery.
"We were patient in our quest to sign him this summer, as he was too, and this news demonstrates the pitfalls of football in general.
"Everyone's thoughts are with George and there is no doubt he will come back from this."
The Rams say Thorne could be out for up to nine months and it comes as a bitter disappointment, just days after signing the promising 21-year-old on a permanent deal from West Brom.