Derby County defender Mark O'Brien will miss the rest of the season after suffering a 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 Derby 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.
"We will support him every step of the way in his rehabilitation work and Mark remains a big part of our future plans."