It felt as if even the airplane was struggling to be up for this trip. Facing Barcelona at Camp Nou is one of the great experiences and privileges in football, one of the thrills of the rarified air of the Champions League in the first place. But there was no disguising it: much of the excitement had petered away when the fixture was reduced to a dead rubber.
It is not quite that - Barcelona still need one point to confirm themselves as Group H winners and secure the advantages that will bring in the last 16 draw - but Celtic's race is run. Four defeats out of five have condemned them to finishing bottom of the section and when the final whistle goes this evening their European adventure will be over until next summer's qualifiers. Given that Barcelona's greatest star, Lionel Messi, is absent with a hamstring injury, even more of the fixture's appeal has been rubbed away.
There is a danger for Celtic in all of this. It is like facing an enemy soldier in the final minutes of a war: the conflict seems slightly unreal but brutal damage still can be inflicted. Barcelona have played 10 home games so far this season and scored 32 goals. When Ajax arrived on Champions League duty they conceded four goals, AC Milan three.
In domestic matches they put two home goals past Real Madrid, three past Sevilla and four past Real Sociedad, Real Valladolid and Granada. Poor Levante were taken for seven on the opening day. Only Espanyol, who conceded only once, and Atletico Madrid, who kept a clean sheet in the Supercopa, can claim to have generally coped with Barcelona's attacking in Camp Nou.
Barcelona have won their last 22 home league games. The absence of Messi is a major advantage for Celtic - although the players will regret his unavailability in both fixtures against the Catalans this season - but they still boast a goal threat from Pedro, Alexis Sanchez and Cesc Fabregas, although the latter is suspended tonight. They have scored nine times since losing Messi.
It was put to Lennon yesterday that for all of Celtic's defensive solidity against Barcelona - since the start of last season they have conceded only four goals against them in three matches - any team entering Camp Nou must acknowledge the possibility of a heavy defeat. "Yes," he admitted. "They also have their own agenda in that they need a point to finish top of the group so I'd imagine it'll still be a strong Barcelona team.
"It's like a cup final almost. You want to enjoy the occasion but you only do that if you play well and get something out of the game. We don't want to come here and get well beaten without laying a glove on them. The players have to have that at the back of their minds. They pressure is off them but they need to go and do themselves and the club justice.
"It was a monumental performance defensively from us last season [Barcelona won 2-1, the winning goal coming in the fourth minute of stoppage time]. Our goalkeeper will again be on call I'm sure. I hope they don't think they are coming here just to fulfil a fixture because they're not. If that's the case, they'll be in for a very rough night.
"People say 'go and have a go at it' but that's a very difficult thing to do against Barcelona when they're at home. We would like to finish the campaign on a high in some form but what I'm looking for more than anything else is a performance. That will take a lot of discipline and concentration, and it'll take a lot of legs as well. Psychologically the shackles are off. The players have an opportunity to go and express themselves when they can. But that's still a very difficult thing to do when you know Barcelona will have the bulk of possession. They work you, so it's what we do when we have the ball. It's about how we affect them when we have it.
"Our players have done the club very proud. This is a great occasion for them. There is pride in terms of trying to finish with as many points as we can in the group, but as far as the players are concerned they've done everything that was asked of them. They did that by qualifying and then by competing. There have been disappointments, sure.
"In the AC Milan game the performance wasn't but the result was. But we created chances that night and at that level you have to take them. We didn't do that and that's why we are where we are."
Emilio Izaguirre is suspended and James Forrest did not travel yesterday. Adam Matthews is in the squad but has not played for two months. Scott Brown and Nir Biton are available again after bans, the captain having been sorely missed during a three-game suspension imposed for kicking Neymar when Barcelona were at Parkhead.
"The three game ban really hurt him and it hurt us as well," said Lennon. "I think we would have performed better in Amsterdam with him in the team. This isn't me going 'it's his fault'. It's just an unfortunate heat of the moment incident which happens to us all. It has hurt him more than anyone because he has missed out on 50% of a Champions League campaign. He will be bursting at the seams to get playing again and I'm really, really glad he is there. For me the team's discipline has been exemplary. It was a nick from Scott and a misplaced tackle from Biton. It's very, very harsh that we have had two red cards when you look at it. We are not that kind of team.
"The players are playing the best football I've seen in my time here. This game is totally different, obviously. It's the elephant in the room when it comes to trying to put a run together. It's Barcelona away. They don't come any more difficult than that."
Celtic know Barcelona well enough by now to know they are in for an exhausting shift. Their concentration must be total, from first second to last. They have just been lauded for seven unanswered goals against Hearts and five against Motherwell. At Camp Nou the initial problem is not getting the ball in the net, but getting it at all.