The fallout from Wednesday night’s controversial Old Firm Scottish Cup fifth-round replay at Parkhead continued as news of the package emerged.
The item, which was intercepted at a sorting office in Saltcoats, turned out to be a hoax, Celtic confirmed.
Celtic chief executive Peter Lawwell said: “Clearly, this most recent sickening event in a long line of threats to Neil and his family is extremely worrying.
“This demonstrates the intensity and pressure which Neil endures as the Celtic manager. However, he has coped with these issues incredibly well during his short time as manager, showing what a strong character he is.
“Indeed, it is extremely sad that Neil has had to contend with such issues for more than a decade, both as a Celtic player and manager.
“No-one in any walk of life should have to live life in this way and those responsible should be condemned.
“It goes without saying Neil and his family will continue to receive every support from the club.
“We know Neil will also have the full backing of all our fans, as he and the club did on Wednesday evening. While these events are very concerning, we would ask that they continue to back the club in this positive Celtic manner.
“While the authorities and other commentators have taken a close interest in recent football events, we would hope they would give similar attention to also condemning such actions.”
The Celtic boss clashed with Rangers assistant boss Ally McCoist at the end of the 1-0 Scottish Cup home win last Wednesday, which saw three Rangers players red-carded.
Ahead of today’s Clydesdale Bank Premier League clash with Hamilton at Celtic Park, first-team coach Alan Thompson took over Lennon’s duties at the pre-match press conference.
Asked why Lennon had not appeared, Thompson replied: “Neil had a lot on his plate in the last couple of days and he thought it was right to stand out of the way for a day or two.
“He has had 24-hour surveillance outside his house two days in a row protecting him and his family and he thought it was right to step out of the way.
“I spend a lot of time with him at work and away from work and I know how difficult it is for his family in Scotland and in Ireland.
“I think it is important that he gets the protection and backing that we give him.”