The sight of blood trickling down the face of Manchester United defender Rio Ferdinand after being hit by a coin thrown by a Manchester City fan marred his side's 3-2 victory in a dramatic Premier League derby on Sunday.
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, and another soon after when a fan ran on to the pitch apparently to confront Ferdinand, detracted from a match full of raw passion and skill.
The police also confirmed that a fan was arrested at the game after racist chanting was heard.
"Man arrested in stadium on suspicion of committing a racially aggravated public order offence after officers hear racist chanting," the police said on their twitter feed.
United manager Alex Ferguson said a spate of recent crowd problems was becoming a cause for concern.
"It's unfortunate that a City fan runs on and coins are thrown," he told the BBC. "That shouldn't happen. The same thing happened at Chelsea, which was masked by all the other things. We could have done without that."
Referring to the incident at Stamford Bridge in October when a steward was injured by angry Chelsea fans when United celebrated an equally dramatic win, the Scot added: "At Chelsea, We had half a seat thrown on the pitch, coins, lighters, and nothing done about it. It's a problem."
A City spokesman said CCTV footage of the incident was being reviewed and the club issued a statement.
"We apologise to Rio. We condemn the actions of the individual concerned. We will support the police with any investigation and hopefully identify the person as soon as possible," it said.
Ferdinand attempted to make light of the incident later on Twitter, saying: "Whoever threw that coin, what a shot! Can't believe it was a copper 2p... could have at least been a 1 pound coin!" although City could find themselves in trouble.
The Football Association issued a statement shortly after the match.
"The FA is investigating the incident in which an object was thrown from the crowd and hit Rio Ferdinand following Manchester United's third goal against Manchester City on Sunday," it said.
"The FA will liaise with Greater Manchester Police and await the reports of both our crowd control advisor who was present at the game and the match referee."
"It is disappointing that after a great game and advert for the Premier League that we are discussing this issue," an FA spokesman said.
"We will work with the clubs and authorities to identify those responsible and support the strongest sanctions available, including life bans.
"We condemn any such acts and it is simply unacceptable that any player is exposed to injury in this way. To witness Rio Ferdinand leaving the field with a cut above his eye to receive treatment is appalling."
In October, Sheffield Wednesday goalkeeper Chris Kirkland was punched in the face by a Leeds United fan during a match.