The final moments of the Manchester football derby turned sour when United's defender Rio Ferdinand was left bloodied above his left eye after being struck by what's believed to have been a coin.
The object was thrown from the crowd after his side's 3-2 victory in a dramatic Premier League game against Manchester City on Monday morning.
Ferdinand was celebrating Robin van Persie's stoppage time winner when he clutched his face and moved away with blood seeping from a wound near his eye.
The incident at City's Etihad Stadium detracted from a match full of raw passion and skill and United manager Sir Alex Ferguson said a spate of recent crowd problems was becoming a cause for concern.
A City spokesperson says CCTV footage of the incident is being reviewed.