Barely an hour after the news broke, fans were expressing shock and disbelief that 'Lenny' had quit after four years.
But many of them were already turning their thoughts towards his successor, with former players David Moyes and Henrik Larsson being mentioned.
Mary Heron, 39, said she was "shocked" to hear the news.
"I've just renewed my season ticket about an hour ago," she said.
"The whole family was at the final game of the season, the trophy presentation, and his speech was very short and sharp.
"It put a wee bit of doubt in a lot of people's minds, especially my own.
"At the time we were too busy celebrating but it was noticeable that his speech was downbeat. It wasn't rapturous.
"We're from East Kilbride and our supporters' group was saying that it didn't think that Neil Lennon was in charge of the money situation with the board. He's only the front man for them, and maybe he's fed up with that."
Mary described Lennon as "a born leader who had proved himself in the European games."
When asked who she would like to see as the next manager, she said David Moyes - "I think he was very poorly treated at Manchester United.
"I'd like to see Moyes come up and start with a clean slate."
Lifelong fan David Beggans, 29, said: "I'm not surprised, to be honest - it's been brewing on the internet the last couple of days.
"It's a bit of a shock. I didn't think it would end as quickly as that. But it's time to move on, I think, isn't it?
"He's been part of the club's history these last 10, 12 years, whatever it's been. It's quite sad. But nobody's bigger than the club, I guess.
"It'll be interesting to see who comes in next. The early speculation has been Henrik Larsson. That'll be good -that will put bums on seats."
One teenage fan, who said his name was Chris, said: "I thought my mate was having me on when he texted that Lennon had gone.
"I can't believe it. He did brilliant for Celtic and we were great in Europe too."
Brian Lawrie, 46, had just been on a stadium tour and had learned the news as he was leaving Celtic Park.
"I'm a bit disappointed. I would have liked him to stay a bit longer.
"But I hope they get the right man to replace him.
"He did well in the European stages - he really brought us on.
"He will be missed. I will miss him."
One 46-year-old fan, who gave his name only as Paul, said: "It's a bit disappointing.
"He's achieved an awful lot - he has exceeded my expectations as a young, ambitious manager.
"However, this is a bit of a shock, because there are a lot of questions with regard to how he handled his personal life in terms of the stress he had.
"But that seems to have dissipated in the last couple of seasons, for obvious reasons."
He estimated that Lennon had brought in at least £78 million more than the club had budgeted for over the last couple of seasons, in things like transfer fees and Champions League appearance money, "but, like in the old days, he was maybe getting peanuts to spend on replacement players."
Matt Boyle, 71, from Erskine, said: "There was a lot of media talk about him leaving, wasn't there? Maybe that got to him.
"I would like the next man to be either Malky Mackay or Davie Moyes. I reckon it should be a Scottish manager.
"Lennon was obviously good for the club.
"But I reckon the other side of it is that, if he's going to go somewhere else, he'll at least be away from all this West of Scotland situation, and all the bad stuff that came with it.
"I was hoping, when the bombs story happened [potentially explosive devices were sent to Lennon and other prominent Celtic fans] he would just walk away from the job, because it must have been an lot of mental stress for him at the time."