Derby midfielder Ryan Connolly will be out for around six months with cruciate knee ligament damage.
The Republic of Ireland youth international suffered the injury playing for the Under-18 side last weekend. He is set to see a consultant next week, with surgery scheduled for the start of September.
"It is a blow for Ryan - we are really disappointed for him," boss Nigel Clough told the club's official website.
"He has our unreserved support and we will do all we can to ensure he returns to full fitness."
Connolly signed his first professional contract at Pride Park in the summer. He made his debut for the club as a substitute last season while still a scholar.
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