Arsenal departed Portugal last night with this tie apparently still very much within their grasp but with a depressing sense of realism pursuing them back to London. The wounds that sapped the conviction from this performance were self-inflicted. Cesc Fábregas's post-match assessment of "schoolboy goals" and checked ambitions was damning.

This can be an insanely cruel competition. The Londoners had arrived in Portugal stripped of the very spine of their first-choice line-up and, in that context, must have been braced for an awkward evening. The irony was that it was the absence of the much-maligned Manuel Almunia of all their injured players that was most keenly felt. Arsène Wenger had talked up Lukasz Fabianski's "fantastic potential" in the build-up but the young Pole sunk without trace here. The Arsenal manager pointed an accusing finger at the referee, Martin Hansson, but it was his goalkeeper's fragile display that cost this side.

This was a horrible occasion for Fabianski. The 24-year-old had appeared nervous in claiming long-range attempts when, 11 minutes in, Silvestre Varela, the right-sided forward of Porto's trident attack – sprung into space afforded him by Gaël Clichy but fizzed his cross waywardly in front of his advancing fellow forwards. Fabianski, however was momentarily disorientated too far from his goal-line and, tumbling back to gather the bouncing ball, only managed to deflect it pathetically into his own net.

It was a concession that defied belief. If the visitors deserved credit for recovering their poise, equalising seven minutes later through Sol Campbell's second successive Champions League goal albeit coming four years since his last, then the implosion six minutes into the second period merely tarnished Fabianski's reputation yet further. Campbell had been retreating towards his own goal under vague pressure, shielding the ball from an opponent, when his heavy touch sent the ball scurrying towards Fabianski. The Pole, panicked again, inexplicably and illegally gathered the back-pass just inside the penalty area.

That was bizarre enough though, having presented the ball to Hansson, the goalkeeper was still dithering while his team-mates disputed the award of the free-kick when the Swedish referee handed it to Rúben Micael. The midfielder placed the ball down and touched it to Falcao, who slid it gleefully into the empty net. Fabianski, crestfallen with his sixth appearance of the season long since spoiled, was singled out for the Arsenal drugs test after the game before departing ashen-faced and speechless. This was a desperately groggy display.

Wenger's fury on the touchline was born less of the reality that Hansson – the referee at the centre of Ireland's World Cup play-off elimination after Thierry Henry's unpunished handball – had inadvertently blocked Campbell from defending the free-kick. Rather, he complained that the centre-half's touch had not been deliberate and the award therefore unwarranted. He was just as apoplectic that the referee had denied his side any time to organise before it was taken. Fábregas, in contrast, had few complaints and his team's inability to rouse themselves to conjure a second equaliser soured the captain's mood further. Any repeat of errors that naive in the second leg and Arsenal will be propelled from this competition. Porto are poor travellers – their 2-2 draw at Old Trafford at the same stage last season aside –and will be vulnerable at the Emirates next month but Arsenal will have to be as ruthless as the Portuguese proved last night if they are to progress. Fábregas, battered and bruised having been targeted by aggressive opponents, will need to be freed from those attentions if he is to exert more influence the second leg.

There were still some grounds for optimism. Only Helton's acrobatics had denied Tomas Rosicky and Nicklas Bendtner when Arsenal's slick approach play did generate attempts at goal. The Czech, too, might have earned a penalty immediately before Arsenal were breached for the second time only for the Swedish official to wave away the appeals. Instead, in the absence of the likes of Robin van Persie, Andrey Arshavin and Eduardo da Silva, the visitors relied upon a returning veteran to eke out what could yet prove invaluable away reward.

Campbell had been playing for Notts County at Christie Park only in September, succumbing to Morecambe in League Two. This was his 200th Arsenal appearance and his first in this competition since scoring in the 2006 final against Barcelona at the Stade de France, with ageing limbs creaking in pursuit of Falcao's burst into the area within the opening 100 seconds. The tackle then summoned by the 35-year-old, perfectly timed to deny the Colombian, was a reminder of his quality.

So, too, was his goal. Arsenal had been behind only for seven minutes when Samir Nasri's corner was flicked on by Thomas Vermaelen, Rosicky nodded back across goal, and an unmarked Campbell thumped a free header into the net. Successive Champions League goals, from Paris to Porto, had the centre-half pinching himself in celebration only for that joy to be doused by the shambolic goal that must now be recovered.