Out-of-contract Griffiths – who is still waiting to learn if new Celtic manager Ange Postecoglou will hand him a fresh deal – fired back earlier this week, claiming he was "at times" given a raw deal by Lennon and then caretaker boss John Kennedy following the Northern Irishman's February exit.

But those words drew a blunt rebuke from his former boss as Lennon said Griffiths failed to join his team-mates in taking up the chance to put in extra sessions during last year's Covid-19 shutdown.

And Lennon issued a further warning to the hitman, telling Griffiths his days at the top are numbered if he does not buck up his ideas.

"He let himself down," he told BBC Scotland's Euros Breakfast Show. "He's on full pay for the three months of lockdown and he comes back, totally out of condition and a stone overweight, when he couldn't afford to be because he hadn't played much football in the previous 18 months to two years.

"We had worked really hard with him to get him back on track and before lockdown, he was in the team and scoring goals and dong absolutely fine.

"Then he comes back completely out of condition, way, way behind any of the rest of them and for me, that was totally unacceptable.

"During the lockdown, the three months off, we opened the training ground. You couldn't come into the building but my fitness coach set up fitness training for the players every day.

"So you would come in, in groups of two, and train for an hour a day – fitness work, every day.

"Callum McGregor, Scott Brown, James Forrest, Greg Taylor, Jonny Hayes – all the lads that lived in Scotland came in. Leigh didn't come in one day, not one day during the lockdown.

"You can take a horse to water but you can't force it to drink.

"It's fine him coming on publicly and saying he's going to do this and that. Well, he's not done it for a long, long time. We started him against AC Milan but we had to take him off after 45 minutes because the game was too quick for him. He wasn't in the right condition.

"Leigh's trying to make it out that it was alright (the way he let himself down) but it's not.

"He's saying, 'I'll prove people wrong' – he hasn't proved anybody wrong.

"He's not at the level of being a Champions League player, he didn't get picked for Scotland – you have to ask why.

"He'll say he didn't play enough games, towards the end of the season I wasn't there, John (Kennedy) obviously felt that he wasn't in the right frame of mind or condition to play for Celtic, which is a very high standard and I don't think Leigh got anywhere near those standards over the last year or so."

Griffiths is hoping Postecoglou will give him one more opportunity to show he can cut it with Celtic.

But Lennon said: "To play at the top level, European level, he's got a lot to do, I think. A hell of a lot to do.

"Even if he is in decent condition, he hasn't got the pace any more. He needs to be slimline, he needs to be sharp – that's his game.

"Bottom line is, if I had a fit and hungry Leigh Griffiths last season, he would have played, because I signed him.

"I brought him back into the team when under Brendan Rodgers, he was nowhere near it. We worked incredibly hard with him.

"If you look at the top players over the years, they all look after themselves. There is no excuse to not be in top condition these days."