Manchester City have apologised to Rio Ferdinand after the Manchester United defender appeared to be hit by a coin as he celebrated his side's third goal in their 3-2 victory at the Etihad Stadium.
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Ferdinand sustained a cut just above the eye in the incident, which is now being investigated by Greater Manchester Police.
"We apologise to Rio," said a Manchester City spokesperson. "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 was able to complete the game and afterwards on Twitter said: "Whoever threw that coin, what a shot! Can't believe it was a copper 2p...could have at least been a £1 coin!"
Sir Alex Ferguson adopted a more serious tone.
"It was disappointing when the supporter ran on the pitch and Rio got hit in the eye by a coin," said the United boss. "The game didn't deserve that."
It was a sad postscript to a thrilling afternoon in which United thought they had established a match-winning lead through Wayne Rooney's double before City hit back in the second half.
Indeed, once Pablo Zabaleta had levelled after Yaya Toure had halved the deficit, it seemed like the hosts would go on to win, only for Robin van Persie's deflected free-kick to seal the spoils for United, who move six points clear at the top.
"We should have been out of sight with the chances we had," said Ferguson.
"But you have to give credit to City.
"They keep going and they score late goals and they have done it against us again. Fortunately we got the last one."
Rooney's double established him as United's leading scorer in Manchester derbies.
The England man is now level with City duo Francis Lee and Joe Hayes on 10.
However, when Ashley Young had a third incorrectly ruled out for offside, City took advantage almost immediately, leaving the game on a knife-edge.
"Intensity, passion, competitiveness...everything was there," Ferguson said. "The incident with Ashley was disappointing because from 3-0 up it went to 2-1 in minutes.
"Football can kick you in the teeth that way. But there will always be controversy in a match like that because there are so many things happening."
It left Roberto Mancini to reflect on another dismal performance from Mario Balotelli.
The Italian was preferred to Carlos Tevez but after an ineffective opening period, showed no sign of improvement immediately afterward, triggering a stark warning from his manager.
"I love Mario but it is important for him to start to think about his job," said Mancini. "He has everything to play well but he can't continue to play like today. We wanted more from him.
"When you have a player that has Mario's quality, you cannot understand why he continues to throw it out of the window. It is incredible.
"I have seen players in my life with fantastic quality. But in the end, they did nothing.
"I don't want Mario to finish like these players. It would be bad for him."
Tevez's introduction certainly sparked a City side - already reeling through the loss of skipper Vincent Kompany with a calf injury - into life.
It was Tevez's half-cleared corner that led to Zabaleta's equaliser, although there was some confusion about where the Argentina star should have been for Van Persie's fateful free-kick given he moved away from his place in the wall to try and mark Rooney, who had become the sixth - and youngest - player to reach 150 Premier League goals.
"I don't know what happened," said Mancini. "I understood before it was dangerous that free-kick and we made a mistake by only putting three players in the wall.
"I called Carlos to come back into the wall but from the bench I was too far away."
The result puts Mancini under pressure given City have also crashed out of the Champions League with the worst performance from any English club in the competition's history.
But, with the spectre of Pep Guardiola continuing to hang over him, Mancini insists City can still retain the title.
"Even though we have lost this game we have shown we can win the title this year," said Mancini.
"It is clear, we need to work hard and stop making mistakes. We can't continue to go one or two goals behind because it is difficult to recover.
"We have a good spirit. Now we need to win four or five matches in a row."