Manchester United's Rio Ferdinand suffered a facial injury from a coin thrown by a fan during the closing stages of Sunday's derby at Manchester City.
The centre-half was celebrating his side's late winner by Robin van Persie but was struck on the head and needed treatment for a bleeding eye.
Ferdinand went on to play the final two minutes as his team beat neighbours City 3-2 to open up a six-point lead at the top of the Premier league.
With the incident being investigated by the police, a Manchester City spokesperson said: "We apologise to Rio. We condemn the actions of the individual concerned.
"We are reviewing the CCTV with the police and our normal security. We will support the police with any investigation and hopefully identify the person as soon as possible."
Ferdinand took to Twitter soon after the final whistle but played down the incident, writing: "What a result, 3 points! Sweet! Whoever threw that coin, what a shot! Can't believe it was a copper 2pcould have at least been a £1 coin!"
United manager Sir Alex Ferguson added: "We were very determined to win but it's unfortunate toward the end that the City fan runs on and throws coins at our players. I don't think you need that.
"The same thing happened at Chelsea which was masked really by all the other things that happened. Two fantastic teams today, I think, and a fantastic game of football and it could do without that."
Van Persie added: "I think it was a great atmosphere, everyone was supporting their team. I missed the thing with Rio but that should not really happen on a football pitch."
There were further ugly scenes in the Etihad Stadium in the final minutes of the match with one supporter running on to the pitch to confront the players.
Greater Manchester Police also revealed that one man had been arrested inside the stadium on "suspicion of committing a racially aggravated public order offence after officers heard racist chanting".