Manchester United's Rio Ferdinand was left with a head wound after apparently being struck by a coin during his team's match at Manchester City.
Ferdinand received a cut above his left eye after it hit him as he celebrated Robin van Persie's winning goal.
The Football Association said it had opened an investigation into the incident, while City condemned it and apologised to the player.
Ferdinand tweeted that he was hit by a 2p coin. United won the game 3-2.
He tweeted: "Whoever threw that coin, what a shot! Can't believe it was a copper 2p... could have at least been a £1 coin!"
Ferdinand was left with blood running down his face after the incident, and he appeared to pick up an object which he later showed to referee Martin Atkinson as he left the pitch.
While trying to recover, a Manchester City fan ran on to the field towards him, but was stopped by Manchester City goalkeeper Joe Hart before he could reach the defender.
In another tweet, referring to Hart stepping in to the fray, Ferdinand added: "Respect to Joe Hart for protecting that idiot who ran on!! Love the banter between fans but there is a point when it goes too far...calm it!"
The man was then taken away by police officers.
An FA spokesman said it would liaise with Greater Manchester Police and await reports from its crowd control adviser and the match referee.
He added: "It is disappointing that after a great game and advert for the Premier League that we are discussing this issue.
"We will work with the clubs and authorities to identify those responsible and support the strongest sanctions available including life bans.
'Identify the guilty'
"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.
"Football in this country has made great progress over recent decades. However, we must maintain the safety and security inside of our stadia and to do this we need to isolate and identify any individuals guilty of offences with strong penalties to act as a deterrent to others."
A spokesman for Manchester City said CCTV footage of the incident at the Etihad Stadium was being reviewed.
He said: "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."
The club added: "[Greater Manchester Police]'s review of all fan behaviour at today's match will be fully supported by the club, through the provision of CCTV footage and eyewitness accounts.
"Following this review, and any associated police action, the club will also apply its own disciplinary policies for any fans found guilty of offences."
Greater Manchester Police (GMP) said no arrests had been made in relation to the incident.
United manager Sir Alex Ferguson said it was "unfortunate" that the object had been thrown.
He said: "That shouldn't happen. The same thing happened at Chelsea, which was masked by all the other things. We could have done without that.
"We can do without that, after a great game between two sides."
In an unrelated incident, police arrested a man on suspicion of committing a racially aggravated public order offence during the game.
The man was held after officers reported hearing racist chanting from the crowd.