Whether the yardstick is the first game in which he was officially in permanent charge, the 3-0 defeat to Braga here in Portugal in July 2010, or the first Group G game against tonight's opponents, Benfica, at Celtic Park in September, the quantum leap in terms of improvement taken by the Scottish champions is undeniable. Tantalisingly, however, it is the final surge for the finishing line which now sits before them, and Lennon acknowledges this is likely to be the most testing stage of the entire journey.
"I don't want the players to get carried away," he said as he settled into the team's hotel in Cascais, half an hour up the coast from Lisbon. "I keep emphasising we haven't achieved anything yet. The aim was to qualify, and it still is, so we have a great foundation. We are going to have to play as well as we have done in previous games to get anything, or to qualify. It's one of those where you're almost there, but you can't quite reach it. That's the real danger when you're in that anxious sort of zone.
"I don't want us to be anxious. I just want them to be concentrated. Just because we've beaten Barcelona doesn't mean we will get something against Benfica. So we have to approach the game in the exact same way we approached the previous games."
That was good enough to take their European record this season to six wins and one draw from eight games in qualification and Group G. Importantly, three of those victories came away from home, debunking the theory Celtic are only a force in Glasgow.
The one defeat came courtesy of an injury-time concession at Camp Nou, and Lennon used that feeling of despair as a spur when Barcelona came to Glasgow two weeks later. Victory that night, coupled with Benfica's defeat of Spartak, threw the group wide open and all four clubs can still go through, and all four can still go out of Europe altogether.
Lennon will not call on the memories of Camp Nou this time around, reasoning that it would not serve the same purpose. "It's a negative, and I don't want to bring up negatives," he explained. "I'm aware that that's a reality, however, and that's why I'm trying to temper everyone's expectation. If we could do it tonight, brilliant. It would mean we could have a nice Christmas and something to look forward to in the New Year. But we'll have to play very, very strongly."
While Celtic stand on the brink of an historic night, Benfica approach the game as though it was a dangerous precipice. The Lisbon club, who only achieved their first win in the section when they played Spartak at the start of the month, will need to add another to keep qualification in their sights.
Lennon senses their desperation and could utilise this. He said: "I think they've only got one bite at it. They know if we draw or win they'll be out of the competition. I'm not saying we're more relaxed about it, but we know we can still take it to the last game in Glasgow."
Like everyone in the group, however, Lennon would prefer to get over the line at the first time of asking. To achieve that, they will need to score and shackle a Benfica attack that has plenty of variation. Rising star Rodrigo and the experienced Lima have the pace to trouble any defence, while the bulk of Oscar Cardozo – who scored a late winner the last time these teams met here in 2007 – is another serious option.
"Cardozo's a handful," recalled Lennon. "He looks a bit cumbersome in his movement, but he's powerful, technically good and his goal record is fantastic. If he doesn't start, they've still got Rodrigo and Lima. They're always on the move and will give our defenders something to think about."
Which is precisely the object of the exercise today when Lennon takes his players for a light session at the National Stadium, where the club enjoyed their greatest-ever moment 45 years ago. "What I would like the players to do is just look around," said Lennon. "I don't even know if there is a tunnel area now, or what it's like. But I think about the snippets of film, when the boys walk out into the sunshine before kick-off. I want them to get a wee sense of history and think about what meant so much to so many people. It might inspire them, I don't know, but it's always nice to go back, anyway."
The presence of the replica trophy awarded to Celtic is here to add to the occasion, though Lennon insists this is not at his behest. He said: "I don't buy into that. It's for the fans, all that sort of stuff. Really and truly, it'll have no relevance come kick-off time."
More pertinent is the presence of Stevie Chalmers, Bobby Lennox and, of course, John Clark, who is now responsible for Celtic's kit. "It's nice having the Lions on the trip," said Lennon, before adding the caveat: "But the expectation level has gone from down there to up there in no time. I want to temper that and water it down by saying 'Listen, we haven't achieved anything yet.' Barcelona was a great night. It's gone now, but we might as well go the whole way and qualify. We've put so much into the group games, and I'd just like the boys to see it through."