The Manchester City forward fell to the floor after one coin struck him as he went to take a corner.
The missiles were hurled towards the Welshman as United fans taunted him during the Carling Cup semi-final at Old Trafford.
Bellamy picked himself up and rubbed his head before carrying on.
City boss Roberto Mancini said: “I didn’t see anything.”
His counterpart, Alex Ferguson, refused to comment on the matter.
The FA are sure to study the stadium’s CCTV footage to try to discover the identity of the fans responsible.
Anyone identified is certain to be banned from Old Trafford for life.
But United will fear the FA could even get tougher and force a game to be played behind close doors — or close part of Old Trafford for a future game.
Earlier. 11 fans were arrested in the build up to last night’s Manchester derby despite police trying to clamp down on troublemakers following the fiery first leg — which led to 18 arrests last week.