Two goals up and looking a sure bet for three points against Dundee United, the Hoops took their eye off the ball in the closing minutes of the game and were made to pay the price by leaving with a 2-2 draw instead of a 2-0 victory.
It was sloppy defending at a stage in the game where clearing their lines should have been the priority, and it goes without saying such lapses in concentration are sure to be punished again on Wednesday night when Barca roll into town.
And, just as concerning for the Parkhead manager, is the injury problems threatening to put his plans for the visit of arguably the best club team in the world into disarray with a number of players doubtful.
Thomas Rogne and Emilio Izaguirre were both forced off with injuries yesterday and are now concerns ahead of the crunch tie in two days' time, as are James Forrest, Gary Hooper and Georgios Samaras.
Lennon remains optimistic about the chances of a few of them being available at least, but he did concede it wasn't an ideal way to be approaching such a game.
"We'll obviously be giving them all every chance," said the Celtic manager. "There are even others, such as Scott Brown, who are also carrying knocks, so it will be a case of seeing where we are in a couple of days' time and taking things from there.
"It is just one of those things though – there is nothing you can do about it. We'll now step up our preparations for Wednesday and hopefully our injury list clears up a little over the next day or two."
Lennon had promised not to allow the forthcoming visit of Barca to influence his team selection at Tannadice, but the omissions of Hooper, Forrest and Samaras perhaps suggested otherwise.
With all three doubtful ahead of the trip to Tannadice, the Celtic manager decided not to take any risks with the trio by rushing them back to face United, with added motivation of seeing his team move clear at the top of the table with a victory to play for.
Having also decided to rest Efe Ambrose ahead of this week's Champions League crunch tie with the Catalan giants, Lennon was forced to introduce the Nigerian internationalist to proceedings in the 20th minute when Rogne was withdrawn due to injury.
It was actually Rogne who came the closest to opening the scoring when he narrowly headed wide from a Brown cross, but that was as good as it got for Celtic in a first half which was devoid of any real quality in front of either goalkeeper.
One of the poorest performers was the man who netted a hat-trick in the previous match against St Johnstone, Kris Commons. Time after time, he surrendered possession too cheaply and it was a similar story with many of his team-mates. There was no purpose to their attacking play.
Likely to have been told in no uncertain terms their performance wasn't good enough by their manager – Lennon has insisted he isn't willing to accept a drop in standards in domestic matches either side of their Champions League encounters – a reaction was expected.
It wasn't, however, forthcoming immediately. It was actually United who were the stronger side in the early part of the second period with Johnny Russell blowing a great chance and Gavin Gunning also missing a good opportunity to open the scoring.
Another forced substitution brought Miku on in place of the crocked Izaguirre, and it was the Spaniard who finally broke the deadlock for his team when he displayed great composure inside the penalty area on 69 minutes.
Ambrose knocked a long ball forward towards Victor Wanyama, who, in turn, played it inside to Tony Watt, who slipped the ball through to Miku. The striker still had a lot of work to do, but he cut inside before slipping the ball into the back of the net.
It came at a stage when Lennon was perhaps wondering where a goal was going to come from, but it immediately sparked the visitors into life and they started to carry much more of an attacking edge thereafter.
Just over 10 minutes later, the second duly arrived. Watt picked up possession and found himself running at the United defence and, once he found himself inside the box, he unleashed a shot which flew by United goalkeeper Radoslaw Cierzniak.
That should have been job done for Celtic.
But, for reasons known only to himself, Paddy McCourt decided he wanted to dwell on the ball, resulting in United breaking up the park in the 89th minute. Kelvin Wilson failed to clear and substitute Gary Mackay-Steven pulled one back from 15 yards.
The impetus immediately switched to the home team, who narrowly missed out on an equaliser when Russell screwed his shot wide, but Ambrose headed Barry Douglas' cross into his own goal to level the scoreline.
It was a sore one for Lennon to take, but the visit of Barca means he cannot dwell on it.