Police are looking for the football supporter who threw a coin at Manchester City's Craig Bellamy during Wednesday's match against United.
It's thought another person has already been arrested for throwing a bottle at Bellamy at Old Trafford.
Manchester United has promised to hand out bans to anyone found guilty of throwing things at players.
But the FA could still fine United over the incidents. The Red Devils won the League Cup semi-final 3-1.
Bellamy was about to take a corner for City during the second-half, when a bottle was thrown at him. Then someone threw a coin which hit the forward on the back of the head.
Police arrested 11 people at the match.
They are looking at CCTV footage from the game to see if they can spot who threw the coin.
The FA is talking to the referee and Manchester United's security team before deciding if it needs to take any action.