It was a wretched, mortifying night for them. No consolation could be drawn from sharing a pitch with footballing masters. Barcelona delivered a rich, sumptuous display, ripe with danger and goals, and they went to town on a dreadful Celtic performance.
The gulf in class between the sides was enormous. The rout completed what has been a bleak Champions League campaign for the Glasgow side: five defeats out of six matches, three goals scored and 14 conceded. There has been no punching-above-their-weight this year, but only confirmation that against Barcelona, AC Milan and Ajax they have been out of their depth. The post-mortems will pore over whether enough money was spent in the summer, or whether funds were squandered on poor recruits. Four of the new signings were on from the start. They were hopelessly outclassed but, then, all of Celtic's players were.
Gone was the organisation, the excellence of the defending, even the spirit Celtic had shown in the three previous games against Barcelona over the last season-and-a-half; a win and two narrow defeats. The defending was too weak, too tame, and there was an absence of belief from the start. Celtic were broken and bedraggled long before the end.
It became a terrible confirmation of the damage Barcelona can do to inferior teams when their play clicks. There was a horrible beauty to Barcelona's superiority. The step-overs, the one-twos, the backheels, the sudden explosion of passing, all of it was murderous for Celtic. The Spanish side have had some mild criticism coming their way after the unthinkable earthquake of consecutive defeats. Oh, to have their troubles. They played like a team luxuriating in their mastery of the ball and enjoying themselves.
They were like butchers, slicing into Celtic down the right wing in the first half and coming at them from everywhere in the second. Adam Matthews, Celtic's left-back, had not played since October 5 because of a broken collar bone and his return was more painful than any fracture. Barcelona constantly attacked him or worked the ball into space for runners behind him. The protection from Georgios Samaras was pitiful, he had one of those nights when he just did not look interested.
All three of Barcelona's goals came from balls into the goalmouth from the left side of Celtic's defence. First Efe Ambrose was caught out and Pedro worked the ball to Alexis Sanchez, whose low shot was saved by Fraser Forster's legs but broke into the goalmouth for a simple finish from Gerard Pique. If even the centre-half could score after seven minutes Celtic were in for a long night. They held out until close to half-time then conceded a quick double. Neymar's quick feet took him free of Ambrose and he rattled over a low cross which Pedro bulleted in at the near post.
Barcelona's front three were outstanding. Pedro on the left, Alexis Sanchez on the right and especially Neymar in the middle toyed with Celtic. When Alexis Sanchez hoisted a pass into space for Martin Montoya, Celtic were all over the place again. Another cross, another perfunctory finish, this time from Neymar.
Neil Lennon has developed a habit of delivering a surprise or two in his selections for the biggest games and there were a handful here. Nir Biton, Derk Boerrigter and Teemu Pukki all started, with Kris Commons, Charlie Mulgrew and Anthony Stokes on the bench. The absence of Commons backfired. Lennon wanted pace in the team but Celtic offered more, and scored, after he came on. But much more was also missing.
Any possession at Camp Nou has to be cherished and nurtured, it can be as rare as osprey eggs, but Celtic were dispossessed far, far too easily and their passing was nowhere near good enough. Pukki was anonymous and lasted only until half-time. Only Forster, Virgil van Dijk and Mikael Lustig were reasonable.
The precision of Barcelona's passing and movement was a joy. Their fourth was another beautifully crafted little work of art: Sergi Roberto to Neymar to Xavi back to Neymar, a shot placed into the top corner and Celtic defenders on their backsides. When Neymar made a mess of Ambrose again he fired a low shot into the goal which Matthews could not clear on the line: 5-0 with more than half an hour left.
Celtic should have had a penalty when Matthews was brought down by Sergio Busquets. Instead the Welshman was booked for a dive. When substitute Cristian Tello's shot then deflected of Van Dijk into the net Barcelona had their sixth. It was marginally more significant for Celtic than the others because it meant the 5-0 defeat to Artmedia Bratislava in 2005 had been eclipsed, at least in terms of goals conceded. No-one had put six past Celtic since Kilmarnock in 1963. When Mulgrew's free-kick was converted by Samaras' glancing header a Celtic striker had finally scored in this season's group.
By then they feared seven. What Celtic can do to domestic teams was being done to them. These two clubs meet so often they have become a little familiar, but this beating took Celtic into horrible new territory.