He could have cruciate ligament damage which would effectively see his season come to a premature conclusion. He has impressed Unitedites with his consistent performances.
The on-loan Blackburn man scored a truly cracking goal against Leicester City just three games into the campaign - it gave the Blades a merited draw.
Kevin Blackwell has spoken about the news which hangs in the balance. He said:
"It isn't looking too good and we will have clarification today (Monday) after he has had a scan at 5pm.
"If it turns out to be cruciate ligament damage he will be out long-term, in the region of 6-9 months. He knew something was wrong straight away when he felt his knee lock and then go backwards."
United do have back-up on the left-hand side of midfield if Treacy's fear does become reality. Kyel Reid is a predominant attacker on the wings; Stephen Quinn can also fill that role if needed too.
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