Manchester United loanee James Wilson faces a lengthy spell on the sidelines after sustaining cruciate ligament damage.
The 20-year-old was injured in training with Derby on Monday and returned to United to undergo tests.
Those have confirmed the severity of the damage and Wilson is set to undergo knee surgery in the coming days.
Derby manager Steve McClaren told his club’s website: “It’s incredibly disappointing news that James has suffered an injury like this.
“Only last week I was saying how much I was looking forward to working with him because he is an exciting young talent.
“I know that he will bounce back from this disappointment. As well as being a very good player, he is also a strong and determined character.”
Wilson scored twice on his United debut against Hull in May 2014 and made 17 appearances for the club the following season.
But opportunities at Old Trafford became harder to come by and he was loaned out to Brighton last season, scoring five times in 28 appearances.
He joined Derby in August and scored his first goal in the Football League Trophy clash with Doncaster earlier this month.