Rangers face disciplinary action from the Scottish Football Association after a paper cup struck Celtic midfielder Stilian Petrov in Sunday's derby.
The object struck as John Hartson celebrated with Petrov
The Scottish Football Association will write to Rangers for their comments, which will be scrutinised by the disciplinary committee.
But Rangers will have to wait until July to discover any punishment.
Rangers say a lifetime ban from Ibrox is the likely punishment for the fan who threw the cup in the 2-1 defeat.
The club are examining CCTV footage of the incident in an attempt to identify the culprit.
And a Rangers spokeswoman added: "The club condemns this type of behaviour."
Referee Stuart Dougal drew the SFA's attention to the incident in his match report.
But any decision on a punishment for Rangers will have to wait until after the season has ended, according to Drew Herbertson, head of SFA disciplinary affairs.
"The next meeting of the disciplinary committee is next Tuesday, which is far too early to discuss this matter.
"The meeting after that will be held in July."
Craig Bellamy, on loan at Celtic from Newcastle, also appeared to be the target of missiles thrown from the crowd when he limped off with a hamstring strain early in the second half.
But the SFA confirmed the incident involving Petrov was the only one mentioned in the referee's report.
Missile-throwing incidents have become a worrying trend in the Scottish Premier League this season, with Celtic and Hearts both being subject to Scottish Football Association discussions.
Rangers midfielder Fernando Ricksen was hit by a cigarette lighter thrown from among Celtic fans as he prepared to take a corner in the last Old Firm derby.
Objects were thrown on to the pitch by Hearts fans during a stormy SPL encounter between Hearts and Rangers at Tynecastle last month.
The SFA took no action on those occasions but wrote to all SPL clubs warning that sanctions could be taken if incidents continued.
Referee John Rowbotham was struck by a coin thrown during this month's Edinburgh derby at Tynecastle.