Lennon spoke calmly, but firmly, about Griffiths's weekend pub episode, when he was filmed standing on a table singing about Hearts going bust. The footage was posted on Youtube. Griffiths attended Sunday's Edinburgh derby and sat in the Hibs ends with his mates.
The singing episode led to the 23-year-old being fined by Celtic and attracting a Scottish Football Association notice of complaint, which could result in a suspension. Griffiths apologised publicly and has also done so to Lennon, but his manager warned him yesterday that his behaviour must improve. When Griffiths joined Celtic in January Lennon had said he was prepared to manage his behaviour, but the first issue with the controversial Scotland striker did not take long to arise.
"I think he was daft, but he's got to stop being daft," said Lennon. "We've dealt with it internally and I wasn't over enamoured with it. I didn't think it was aggressive and nor was there any foul language used. But there's a code of conduct he has to adhere to now. He did cross the line in that aspect. It is a bit of fun, but in this day and age, with modern technology, people want to drop you in it. A lot of people would have found it funny, but I'm sure a lot of Hearts supporters would have found the opposite. Look, we've all been guilty of doing daft things at times, but that's his one strike. I don't want him doing any more."
Lennon was asked if Griffiths understood that his behaviour has to improve. "He has to, otherwise his career at Celtic won't be as long as he maybe wants it to be. I think he knows that, whilst he may feel he hasn't done a lot wrong, I think he understands the reasons why we are disciplining him. I want him to make the headlines for the right reasons, i.e. footballing reasons. He's started his career here very well, but I don't want him to get carried away with that."
Griffiths, who has scored five times in nine appearances for Celtic, will receive a formal letter from the club detailing his internal punishment.
"I haven't hammered him or overemphasised it - you have to give players a bit of space. But I don't want to have any more conversations about it, put it that way. He needs to understand that there is a certain example he has to set.
"Players have got to live a bit as well. But I don't want him singing with Hibs fans about other clubs. He's a Celtic player and his whole focus should be on Celtic. He knows he's done wrong. But we've had players here who have done a lot worse than that.
"He had his problems at Hibs as well. He's got to just tidy a few things up. It's not the worst thing in the world, but it's obviously derogatory to some people, and some people are sensitive to these things, particularly in Scotland and particularly when it comes to Old Firm players.
"[Otherwise] he has been the consummate professional since he's come in. We've dealt with it, we've drawn a line through it, and we will go and support him at the SFA hearing. But I'm not going to have these conversations with him time and time again, I think he understands his role now at the club."
Griffiths' SFA hearing, on a charge of not acting in football's best interests, is scheduled for April 24.