Celtic chief executive Peter Lawwell has highlighted the "mitigating factors" surrounding manager Neil Lennon at last week's Old Firm match.
Lawwell said Lennon's conduct "slipped below the high standards at Celtic".
But the chief executive added: "He's under the normal pressure as a Celtic manager to be successful and win games.
"But, other than that, he's got to put up with live ammunition going through the post, bomb threats, 24-hour surveillance. It's a lot to take on."
Lennon became involved in an angry exchange with Rangers assistant manager Ally McCoist on the touchline after the final whistle of last Wednesday's Scottish Cup replay, which Celtic won 1-0.
And Lawwell was speaking after attending a summit to discuss Old Firm-related disorder with his Rangers counterpart Martin Bain, Strathclyde Police, Scotland's football authorities and Scotland's First Minister Alex Salmond.
"Neil reacted to a particular situation, didn't instigate, reacted to a particular situation in a way that he regrets," said Lawwell.
"He very quickly, immediately apologised to the Celtic board and the matter is now closed.
"He then also had a meeting with Ally McCoist and the matter for the two of them was closed.
"There is no way we can condone Neil's behaviour and I think he would agree with me on that that night. He slipped below the very high standards we have here at Celtic and he recognises that.
"But I think in terms of mitigating factors, you have to understand what's going on in his life at the moment."
First Minister Salmond said at the summit: "We've had reports of quite disgraceful threats against the Celtic manager.
"We deprecate such behaviour and the police will trace and deal with the perpetrators through the courts."
Meanwhile, Celtic and Rangers have jointly issued a six-point plan aimed at addressing problems arising from the Old Firm fixture, including measures to tackle sectarianism, drink-related offences and domestic violence .