There is resentment at the performance of the referee, Alberto Undiano Mallenco, and the amount of grappling that he allowed in the Juventus penalty area. A feeling of injustice can, after all, be a handy motivational tool for a manager, but the club is primarily acting out of a sense of injustice. Even so, other incidents had to be dealt with more swiftly and conclusively.
No manager would, after all, want acrimony to settle within his squad. Immediately after the 3-0 defeat at Celtic Park, Kris Commons was critical of the performance of his team-mate, Efe Ambrose. The defender was at fault for two of the Juventus goals, misjudging the flight of the ball for one and being caught in possession for another, and Commons could not conceal his irritation. The game's convention is for team-mates to support each other in public, and the remarks seemed blunt, even if the raw emotion of the occasion could have excused them.
"I have dealt with it internally," Neil Lennon said of Commons' remarks about Ambrose. "There will be no disciplinary action taken. Kris said things in the heat of the moment. But it shows to me that he really cares about the club and team. He knows that he shouldn't be singling people out. We are a team, we are a unit. He has learned from that and we move on. I would imagine he has apologised privately."
Lennon, too, was the subject of sharp comments for selecting Ambrose, since the defender had only returned from Africa Cup of Nations duty on the morning of the game. There was some logic in that censure, but Ambrose has always been capable of moments of calamity, however fit or drained he might be physically, and the more glaring error was the free header from which he failed to score in the second-half. Lennon was less forgiving of the latter, but on reflection he would not change any of his decisions.
"Efe is all right," the Celtic manager said. "He was obviously very apologetic after the game. He made basic errors but it coincided with his situation. If I had my time over again, I would still pick him for the game. No-one would have said a word if he had scored with the header to equalise, which he should have done. That's nothing to do with tiredness or fatigue. It's just a bad miss. If he makes it 1-1, no-one would have been questioning the selection or saying it was madness to play him. That's the fine lines that managers walk on sometimes.
"Efe is adamant he should have played. It wasn't really a massive decision for me. He has been fantastic for us. He's added a different dimension to the way we play, particularly in Europe. Bringing him in alongside Kelvin Wilson has made us defensively a very good outfit in Europe, with the pace that they bring. His performances away from home in Barcelona, Lisbon and Moscow were as good as any centre-half I've seen at the club for a long, long time. Wilson has grown with that partnership and he had a fantastic performance on Tuesday."
Perspective might have been a victim of the scoreline, since the weeks between the draw and the tie itself had allowed optimism to grow among the Celtic support. Losing three goals, and feeling let down by the officials, is chastening, but keeping the result in context is important none the less. Celtic still have the championship to win, and are in the quarter-finals of the William Hill Scottish Cup. Even the second leg against Juventus carries meaning, despite the extent of the Italian side's lead.
"I'm very proud of the team," Lennon said. "The scoreline is harsh on us but that's the reality of football at that level. Juventus are an excellent team. What I want to do now is win the game in Turin if we can and salvage some pride, if you want to call it that, in the tie. It would be a fantastic effort to win in Italy and it would also equal the club record of wins in Europe for a single season. So there is still plenty of motivation for us going to Turin.
"Listen, if you had said to me last 16 at the start of the season, I would have been saying, 'we're talking about miracles here'. The group was strong, but I felt that if we could make it through the qualification we had a decent opportunity to progress in the competition and we've done that. The difference at the end of the day is £100m, and quality counts sometimes."
The rehabilitation of the squad's mood begins at Celtic Park today against Dundee United. The visitors are in a resurgent frame of mind themselves, since Jackie McNamara succeeded Peter Houston as manager. Lennon and McNamara are former team-mates, and the Celtic manager is wary of a sprightly United, particularly because of his admiration for the new manager.
"[Jackie] was a very intelligent player and a very intelligent lad," Lennon said. "It doesn't surprise me the progress he has made on the coaching side of things. He's got a good support mechanism. His father was a great player, and with Simon [Donnelly] and Darren [Jackson], there's vast experience of the game there [at United]. It's always a dangerous game, Dundee United. We played very well two or three weeks ago to win at home and I'm hoping for a similar reaction, particularly after the other night."