Messi, Suarez, Neymar and company had just tamely surrendered in Camp Nou to Alaves, whose last venture in La Liga ended after a single season 10 years ago.
In one sense it was the right result.
La Diada itself commemorates a terrible defeat: the day when the Barcelona garrison surrendered to the armies of King Philip of Spain and Catalonia lost its independence, 302 years ago now.
Barca fans are well-known for their sense of history, but losing to Alaves, who have spent many more years in the third division than the top flight of Spanish football, was definitely not on their agenda.
At the final whistle, the faces of the fans all round Camp Nou betrayed more than a sense of shock. It was incomprehension.
There is never a good time to play Barcelona, but as far as Celtic are concerned there could be worse occasions.
Coming off a 5-1 thumping of Rangers the players must be feeling good about themselves.
The worst thing when facing Barcelona is to travel with a sense of inferiority. And although Andres Iniesta issued the inevitable public declaration Celtic should ignore the Alaves result, the fact is that the Catalans do have a couple of worries.
One is Alaves got a result with steady defending across the back and quick breaks from midfield. They had a five-man defence but were never drawn too wide. When the breaks came they delivered crosses precisely and quickly into the danger areas.
Alaves manager Mauricio Pellegrino, who Liverpool fans may remember as a defensive coach under Rafa Benitez, had obviously done his homework. Their two goals were no flukes, in fact they would have scored three but for a very close offside decision.
Luis Enrique has admitted he underestimated the opposition. Having rested all his main players he then brought on the cavalry with half an hour left, thinking the opposition would crumble. Maybe it was the new blonde hairdos but neither Messi nor Neymar looked like himself. Nor did an oddly subdued Suarez have much of an impact.
Barcelona continued to balloon shots over the bar, and when Messi finally did get into one of his classic scoring positions he screwed his shot a yard wide.
The other concern for Luis Enrique is his defence.
Their entire back four and the goalkeeper were rested against Alaves, and Celtic will obviously face a completely different line-up. Among other things that will give Barcelona greater width in attack. But there is no disguising their uncertainty in goal.
Claudio Bravo – who never seemed totally reliable – has now departed, leaving Marc-André ter Stegen and Jasper Cillessen in contention. Ter Stegen suffered a small leg injury in training that kept him out against Alaves and Cillessen did not seem comfortable, particularly on crosses.
Both are internationals, Cillessen for the Netherlands, Ter Stegen as a deputy to Manuel Neuer for Germany, but neither of them have the height and reach of today’s top keepers. Both trained with the team yesterday. Ter Stegen is favourite to start.
Delving back in the archives Madrid’s sports paper Marca revealed that Barcelona last conceded four goals in consecutive home games over 30 years ago and they have now let in two against both Betis and Alaves, hardly top-class opposition.
They do have a weakness against pace, so there is a chance that Scott Sinclair or Moussa Dembélé – such a revelation against Rangers – could cause problems, assuming Celtic can get some possession in midfield.
But statistics need to be kept in perspective. Barcelona did let in two against Betis – and Betis had other chances – but they also scored six, including a Suarez hat-trick. Suarez has 17 goals in his last seven league matches.
Neymar was away at the Olympics with Brazil, and there has been talk of him being distracted. His first record is due out – nothing to do with goals this time, it’s a song entitled Neymusico – and some fans have suggested that he ought to stick with what he’s good at.
So there may be scope for headlines about Barcelona lacking rhythm or their strikeforce being out of tune.
On the other hand Iniesta is still the maestro in midfield, and with him conducting affairs that is where tonight’s match is likely to be decided.