Derby defender Mark O'Brien has been ruled out for the rest of the season after suffering a serious knee injury in training.
O'Brien underwent reconstructive surgery on Wednesday afternoon after damaging his anterior cruciate ligament the previous day.
"It's a big blow for Mark and we're all devastated for him because his form and progress have been big positives in the first half of the season," said manager Nigel Clough. "There was no-one around him in training when he went down injured, so there's a huge element of misfortune about how it happened.
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We will support him every step of the way in his rehabilitation work and Mark remains a big part of our future plans."
O'Brien recovered from surgery to repair a heart valve problem just over two years ago and then an ankle operation to become a regular in Derby's matchday squad. The 19-year-old has featured in 21 of the club's 24 league and cup games this season, making 16 starts.
Clough added: "Mark is a remarkable character and he's had a few setbacks in his short career.
We know just how strong he is and he'll be back from this."