Harry Chapman has been ruled out for three to four months, Sheffield United boss Chris Wilder has confirmed.
The on-loan Middlesbrough midfielder suffered a freak training ground injury last week, damaging ligaments when his ankle gave way under him.
"It was a disappointing week really," Wilder said. "I think it knocked the players back, just the manner of what happened.
"It was an innocuous situation, no one anywhere near him.
"It's desperate news for him because he has become quite an important player for us. He was going to play on Sunday against Bolton.
"These things happen, the boy will get over it. He'll have the best medical attention from his parent club and whatever we can do as well as a football club.
"It was just the manner in how it happened really but it's not the first and it won't be the last time something happens on the training ground.
"It's an unfortunate situation and he's going to be out for quite a while.
"He's ruptured a ligament in his ankle so it's going to be three or four months.
"He's going to need an operation. It takes him well into the New Year."
Chapman's loan expires at the end of the month and Wilder confirmed he would have looked to extend the deal through the rest of the season.
The Blades boss will closely monitor Chapman's recovery over the next few weeks before deciding on whether to continue with the loan.
He said: "We have an option in January to extend it. We'll get all the details and work something out.
"We'd like to keep the player but obviously if he's not going to be involved until really late on in the season, we're going to make that decision."
Chapman has made 11 appearances for the Blades so far this season, scoring four goals, including a hat trick in the FA Cup win over Leyton Orient.