Sheffield United fear summer loan signing Keith Treacy could be ruled out for the rest of the season.
The Blackburn winger, 21, who agreed a six-month loan deal at Bramall Lane in the summer, sustained the injury during Saturday's defeat at Swansea, where he was forced out of the action midway through the first half.
Manager Kevin Blackwell said: "It isn't looking too good and we'll have clarification after he has had a scan on Monday."
He added: "If it turns out to be cruciate ligament damage he will be out long-term, in the region of six to nine months.
"He knew something was wrong straight away when he felt his knee lock and then go backwards."
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