Barnsley striker Iain Hume is being monitored in a hospital high dependency unit after being diagnosed with a fractured skull.
The Canada international was forced out of Saturday's stormy derby clash following a challenge with Sheffield United skipper Chris Morgan, and his condition later deteriorated.
Tykes manager Simon Davey said: "He got caught in the face with an elbow. He couldn't move his neck. His neck was stiff and his cheekbone's not in the same shape it was when he started the game."
Former Barnsley defender Morgan was booked by referee Andy D'Urso after the 35th-minute incident.
Davey added: "At the time I thought when somebody raises an elbow and catches you in the face, it usually is a red card, but you're talking about playing Barnsley and at the moment that doesn't happen."
United have responded to news of Hume's injury in an official club statement, which read: "Sheffield United are saddened and concerned to hear that Barnsley striker Iain Hume is being monitored in a high dependency unit following last Saturday's fixture and we wish him a speedy and full recovery."
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