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.