The first-leg loss to Shakhter Karagandy need not prove fatal - Lennon and his players will fancy their chances of overturning the two-goal deficit in the return at Parkhead on Wednesday - but it was further proof of the old adage that, as Celtic manager, you are rarely more than two games away from a potential crisis and never far from a chorus of voices ready to tell you so.
As a lifelong fan, the club is in Lennon's blood but it would be a mistake to think it has led to him leading a carefree existence where he wakes up every morning with a spring in his step eagerly awaiting the new day. Instead there have been regular bouts of insomnia as he frets about the future, motivated by a fear of failure as much as by a desire to savour more success. Having taken Celtic to the Champions League last 16, and with no real pressure domestically due to Rangers' extended spell in the lower leagues, it is difficult to imagine a manager in Scotland with greater job security than Lennon, but there was a candid admission that it is that prospect of being told he is surplus to requirements that drives him on.
After a sticky start, there have been more highs than lows, although Lennon does not see it that way. It is the defeats that linger longest in his mind. Even when his team recorded the best result in the club's recent history by beating Barcelona, the enjoyment was fleeting. The need to constantly strive for improvement and look to the next challenge made sure of that, anything to avoid straying into crisis territory.
"It keeps you thinking," he said. "It gives you some sleepless nights sometimes. The one thing I fear is the sack and that's what drives me more than anything. Obviously, producing a good team, good football and winning silverware is added to that. But the fear of the sack is the thing that motivates me more than anything else.
"When we beat Barcelona, I was thinking, 'we've got to qualify now'. Looking back, maybe I should have made more of it at the time. But that's just not the way I am. There are no doubts the lows outweigh the highs in the job . . . for me, anyway. Some managers might be different and really enjoy the moments. I do have that enjoyment but I try to keep a lid on it. Maybe it means more to me because I've been a player here and I do enjoy the club. But there are times when it's difficult.
"Look at Walter [Smith]. I'm sure he had lows in his career as well as a lot of highs. It'll be same for Martin [O'Neill], Gordon [Strachan] and Sir Alex [Ferguson]. It's how you deal with it. That's what fascinates me more about them than their success - how they dealt with the huge lows they experience."
Smith, Lennon's one-time Rangers adversary, once noted that it was the prospect of failing that drove him on. Lennon can empathise. "That's the same with me. It's a horrible thing, though. It really is. The fear of not qualifying for the Champions League has given me a lot of thought . . . since April really."
That focus will intensify in the days ahead. Celtic, of course, have a league match at home to Inverness Caledonian Thistle to contend with first but it is what happens on Wednesday that will shape their season. Having lost three key players in Gary Hooper, Victor Wanyama, and Kelvin Wilson over the summer, qualifying for the group stage from two goals down would, in Lennon's eyes, be more of success than reaching the last 16 last year.
"It would be fantastic. It would be a bigger achievement in my mind," he said. "These are difficult times for us in Scotland and to bring that little bit of quality to the country [would be great]. It gives the players that bit of development to compete against the best. And it would give the fans a lift as well. It would be brilliant."
The result will have a huge bearing on Lennon's next foray into the transfer market. Targets who may tempted by the prospect of Champions League football may not be as turned on by the Europa League, especially with little competition domestically and no Old Firm games. Lennon would ideally like to sign three more before the window closes a week on Monday but knows that circumstances will dictate.
"Players might not want to come if we're not in the Champions League," he conceded. "So there is a lot riding on Wednesday, but there always was. Now there's probably a little bit more. There are loads of players [we are looking at]. I know I've been saying that for weeks now. There is this perception that we are gambling [by not having signed anyone for the play-off tie] and the board has gambled, but that's not been the case at all. We have made bids for players and we have been knocked back.
"It's frustrating we couldn't get players in before this tie. But is it any easy thing to say that you can buy your way out of trouble? I don't like to do that and I never have. That's an easy excuse to make yourself look better."