The manager can dismiss the outcome, because few teams are able to live with the quick incisiveness and brutal dominance of the Spanish champions. The regret lives on from other matches and incidents, moments that draw together to reveal a campaign in which Celtic came up short of the necessary quality and guile.
There were elements of misfortune in goals being deflected past Fraser Forster, but a 6-1 defeat will always sting. Celtic battled gamely, but the defensive solidity they needed to build on was missing and it quickly became a chastening experience. Lennon's side achieved glory against these opponents in Glasgow last season; this was a harsh reminder that the elite level can also bring despair.
Early performances were promising, and losing 2-0 in Italy against AC Milan in the opening tie will still rankle when the away team offered so much in the game. Yet Celtic could not punish opponents or hold them at bay, and the result was a series of disappointments.
There will always be an element of frustration for a team in Celtic's position, since every summer demands an element of rebuilding. Lennon must, for instance, wonder how a central defence of Kelvin Wilson and Virgil van Dijk would have performed, of how Victor Wanyama would have brought greater authority to his midfield, and of how Gary Hooper would have offered more threat and penetration up front. This has been a time of transition, since the players brought in as replacements have yet to prove their worth.
Van Dijk stands alone, though, since he has improved throughout this season. Tall, strong, adept, he is also more composed and less impetuous than Efe Ambrose, his centre-back partner who reminded again that he is one rash touch away from throwing his defensive colleagues into turmoil.
The back four could not contain Neymar, but that is no indictment when the attacker possesses such exquisite skills and devilish intent. None the less, Mikael Lustig was culpable in the first-half goals that Celtic conceded, and Adam Matthews performed like a player who was returning after two months out injured.
This was draining for Celtic. Energy and spirit were sapped by the galling experience of being casually and ruthlessly despatched by the Spanish side. Morale can be restored - Celtic's recent domestic performances have been imposing and even an improving Hibernian side ought to be overcome at home on Saturday - but the Champions League is an elite competition and Celtic did not rise to its demands in this campaign.
They needed the replacements they signed last summer to develop into players capable of performing at the same level as Wanyama and Hooper, and to provide the qualities that the team has lacked in Europe this season. A shrewd, imposing defensive midfielder is required, so can Nir Biton assume that role?
A clever, industrious and quick striker is needed to play up front on his own away from home and allow Celtic to develop play in the opposition half, so can Teemu Pukki take that responsibility?
In Georgios Samaras - for all his idiosyncrasies - and Kris Commons, Celtic have players who can create and be occasionally devastating in the final third, but can Dirk Boerrigter provide the pace and penetration on the flank to enhance Celtic's counter-attacks?
Questions abound, and Celtic fans will fret about the future of Forster, particularly if the goalkeeper can become England's No.1 during the World Cup. Decisions need to be made on the futures of Samaras and Joe Ledley, two players Lennon would no doubt like to keep but whose roles in the squad must be weighed up against their wages and the requirements to strengthen other areas.
Lessons have been learned in this campaign, in particular that there is very little margin for error in Celtic's recruitment policy and performance levels when they step up to the Champions League. The best of the qualities that carried Celtic into the group stages last season have not been replicated, but that does not mean they cannot be again next season.
Celtic need refined, and need the additions to the squad to develop and mature. It will take time, and shrewd signings have to be made next summer. This was a campaign that the team could not find the means to cope with.