Gary Naysmith faces a lengthy lay-off after undergoing surgery to repair cruciate ligament damage.
The Sheffield United left-back was injured in Sky Sports' televised match against Burnley and is expected to be out for around seven months.
"We hoped that there was no serious damage to his knee," said manager Kevin Blackwell
"Gary has undergone surgery on his cruciate ligament and will be out for six or seven months."
Blackwell admitted his disappointment at the injury as Naysmith had started 37 games for the Blades this season and was one of the most consistent performers in the club's promotion push.
"It is extremely disappointing because he has been such a solid player for us all season and to lose him at this absolutely crucial stage is desperately bad luck for him and the club."