Manchester United's Rio Ferdinand suffered a facial injury after a fan threw an object during the closing stages of Sunday's derby against 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.
United manager Sir Alex Ferguson said: "We were very determined to win today 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."
There were 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".