Celtic assistant manager Johan Mjallby has claimed it was a "dark day" for Scottish football after manager Neil Lennon was attacked by a Hearts supporter during the club's 3-0 win at Tynecastle last night.
Celtic's win means the race for the title will go down to the final game of the season, although the comfortable victory was overshadowed by the attack on Lennon.
Mjallby said that Lennon was "shaken" and that the attack could have been much worse.
"What has happened to Neil is a dark day for Scottish football and I've never seen anything like it. He is shaken but OK," he said.
"I saw someone coming and we didn't react at first, we were obviously shocked and the guy could have had anything in his hands."
Lennon was approached by the fan after Gary Hooper had put the visitors two goals ahead.
Before the game could restart the fan clambered from the Hearts section of the main stand and made a beeline for the Celtic boss, who was on the touchline, leading to an intervention by police and stewards after the fracas ended. The supporter was then taken away by police.
The fracas followed a string of previous off-field problems for Lennon. In January a package addressed to him containing bullets was intercepted at a sorting office in Co Antrim.
Then in March, the club confirmed Lennon was under 24-hour security surveillance after a suspicious package was intercepted at a Royal Mail sorting office in Saltcoats, Ayrshire, and on April 19 it emerged that Royal Mail intercepted a total of two "viable" parcel bombs addressed to Lennon.
Mjallby has now questioned how much more Lennon can put up with, even though he has a tough exterior.
"I haven't really had time to speak to Neil, he has a strong character, but how much can a guy take? We hope he is going to be in a good mood later on tonight, but I really dont know at this given moment. We have to look into it. This can't happen again."
Hearts boss Jim Jefferies believed the feeling in the ground all night suggested something could happen during the match.
"We can't condone that, if it is getting to that stage it has got to be seriously looked at," he said.
"It seemed to be a very hostile atmosphere tonight for some reason, I just felt all night there was something ready to spark it off."
The relevant authorities were quick to issue statements regarding the incident as SPL chief executive Neil Doncaster said: "The SPL deplores the actions of the individual involved, this type of behaviour has no place in Scottish football.
"The SPL will undertake a full investigation and will be reviewing reports from the SPL match delegate, match officials, the police and the clubs."
Hearts vowed to give their full co-operation to the authorities as they attempt to rid the sport of such incidents. A club statement read: "An investigation into events which took place in the second half of tonight's game against Celtic is now under way.
"These sorts of actions have no place in the game. We'll give our full support to the stewards and police authorities in the ensuing investigation."