It is a stern enough test to face Barcelona with 11 men, let alone 10. Celtic discovered precisely that. Anger was the overwhelming emotion in Glasgow's east end after the dismissal of the Celtic captain, Scott Brown, proved the defining moment of a taut encounter.
Brown's departure on the hour was sufficient to hand Barcelona the impetus they needed to secure victory by a single Cesc Fábregas goal. It was a success they barely merited from a match that regressed into the realms of the bad tempered for the closing 30 minutes.
Thereafter, discussion inevitably surrounded Brown's clash with Neymar. Brown was punished for a needless kick after a foul on Barça's Brazilian forward, a matter that Neil Lennon regarded as unduly harsh.
"I think it was very soft," said Celtic's manager. "If there was any contact, it was minimal. There was a lot more going on in the game that warranted heavy punishment. I don't think Neymar does himself any favours with the way he behaves at times. I don't think I need to justify my defending of my captain; I think the sending off is uncalled for and unnecessary."
Lennon could be happier with events in Amsterdam, where a draw between Ajax and Milan keeps Celtic's hopes of progression to the last 16 alive.
The Barcelona coach, Gerardo Martino, used universal managerial language when assessing what occurred between Brown and Neymar. "I couldn't see it," insisted Martino, before refusing to comment on Neymar's behaviour.
Celtic hope could be derived from recent history. Last November's win over Barça here created shockwaves all over Europe, while earning Lennon and his players the kind of glowing recognition that could never arrive on account of dominance in Scotland.
The ominous flipside to the events of last season for the hosts was that Barcelona could be forewarned and forearmed for their latest visit to Glasgow. Martino's team have won their opening seven La Liga matches in creating a club record; Lionel Messi was missing for this Group H tie but a front three of Pedro, Fábregas and Neymar looked a suitably potent antidote to that.
The opening hour was noticeably low on clear-cut chances, in itself a credit to Celtic. That much apparently frustrated Neymar, with the striker fortunate to escape without a booking for throwing himself to the ground, 20 yards from the Celtic goal, after 25 minutes.
The former Santos player slid the ball narrowly wide, five minutes short of the break, when meeting a Dani Alves cross. That served as a reminder to Celtic, who had impressively kept the Catalans at arm's length, as to the value of concentration. Xavi proved likewise, with a free-kick that prompted Fraser Forster into an unorthodox save only seconds before half-time.
A hitherto tame contest was to explode into life 40 yards from the Celtic goal. Brown fouled the marauding Neymar, in itself not an incident worthy of a red card. Where the Celtic midfielder erred was offering a kick towards an opponent who would never be slow in making the most of it. This Champions League has not been kind to Brown, who fouled Mario Balotelli in the San Siro a fortnight ago in conceding the free-kick that led to Milan's second goal of a 2-0 win over Celtic.
Brown gave little by way of protest before heading to the dressing room but the home support more than made up for that, with Neymar's every move thereafter met by enraged noise.
Amid this almighty din, Lennon's response was to introduce James Forrest and Beram Kayal from among his substitutes. That almost paid off immediately as Forrest's half-volley forced an excellent Victor Valdés save. From the corner, Charlie Mulgrew sent his free header agonisingly wide.
Barça predictably supplied immediate punishment. The substitute Alexis Sánchez crossed for Fábregas, anonymous to that point, who floated a header past the stranded Forster. Only brilliance from the goalkeeper prevented Barcelona from adding late gloss to their win. A bigger margin of victory would have been unduly cruel towards the hosts, who had suffered quite enough after Brown's premature departure.