A thigh injury will keep the forward out for two to three weeks, and the reaction among the Catalan media was to agonise over the consequences. The message from the club was that the team would cope without its talisman, and both points of view are revealing. Even so, Messi dominated the build-up to a game that he will not participate in.
With his small, thin frame, infallible technique and glorious ingenuity, he is typical of this Barcelona side, but has also superseded it. The Spanish champions are renowned for their possession-based game and an elegant but devastating superiority built on a team ethic, yet Messi undoubtedly stands alone, like a messianic figure.
The manager, Gerardo Martino, said bluntly that Messi's absence "cannot be an excuse" against Celtic tonight, while the player most likely to take his position in the forward line, Cesc Farebgas, talked of the need for the other players "to step up". The gathered Spanish journalists never seemed wholly convinced, since they kept returning to the topic with both men at Celtic Park last night, exposing the uneasy balance at the heart of Barcelona: a club that considers itself to have a unique identity to last the ages but that also relies upon one of the two best players in the world.
"You always miss a player of his talent," said Fabregas. "He can win a game at any moment. But you cannot always put your head down and feel sorry for yourself because you are missing a great player. We all have to step up and make sure that we don't feel his absence. I'm sure that, with the quality players we have, we can manage it.
"Without Messi, we will all have to take more responsibility, because he is the player who scores more goals, creates more assists than anyone else. If I am in the line-up, I might have more offensive responsibilities. We have to try to adapt the situation. All the strikers - myself, Neymar, Pedro, Alexis - have to focus on doing the job of strikers: scoring goals and providing assists."
There is an uneasy mood around Barcelona. The team has won all seven of its La Liga matches this season, often overwhelming teams in the opening half hour, as well as winning the Spanish Super Cup and defeating Ajax 4-0 in their opening Group H match. Yet Martino's tactics are scrutinised for subtle changes to those of his predecessors, Tito Vilanova and Pep Guardiola, while Fabregas faced a range of questions about his own contributions.
Under Martino, he has remained influential, but the Argentinian has also sought to introduce variety to Barecelona's play. It is still based on ball retention, but there is now a greater willingness to forgo patience and precision to try to hit teams on the counter-attack, or to play longer, more direct passes if opponents need to be broken down a different way.
The 4-0 victory over Rayo Vallecano last month was the first time in 317 matches that Barcelona's opponents enjoyed higher possession. Despite the arrival of Neymar being the only major personnel change, Barcelona are in a period of transition.
"It's true that some people think I could do better, help the team even more," said Fabregas. "Now I feel confident, I feel in good shape, the manager trusts me and that is important. I'm in a good moment. The main thing I have to do is be more consistent. Tata is very coherent. He is reasonable and he is very clear to the players. We should introduce some changes that he has suggested to complement the way that Barcelona has always played.
"[Neymar] is a very talented player. He's very quick in a short space, he has the skills to get away very easily. He's a star in the making, this boy will have a lot to say in the future of football, we will be talking of him as one of the greatest. Neymar can make a difference. Celtic are very strong defensively, but he can handle the pressure unlike many players of his age."
Fabregas said that he admires the "passion" of Neil Lennon and having played at Celtic Park twice before he understands that the arena can influence the opposition. Even so, there was little talk of avenging last season's 2-1 defeat in this competition. The approach of both teams is likely to be similar, which means that the visitors must take advantage of their long periods in possession of the ball if they are to avoid another difficult evening.
"We won't obsess about what happened last year," Fabregas said. "We have learned the lesson. We had good possession, we had good chances - but they had two chances and managed to score two goals. Of course we'll attack again, but we cannot leave room for chances at the back. We know that we should be wary of set-pieces and counter-attacks, because they are the main threats. But we still need to play in our usual way."
Martino expressed the same opinion, and there was no sense of a manager under pressure. The responsibilities of managing Bareclona, for now, rest comfortably on his shoulders. "This is a new season," he said. "This is Barcelona and we have the requirement to always reach the final of every competition we play."