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If ever a moment proved the value of experience on the European stage, it came when Gabriel Martinelli looked up in the 93rd minute to see Martin Odegaard and Bukayo Saka in space across the field. Five Porto players were moving between the ball and his teammates. Get that pass right and Arsenal might be returning to London a goal to the good. Get it wrong and, well, the Estadio do Dragao will not soon forget what happened then. As he so often did on Wednesday night, Otavio snuffed out a promising attack from the visitors.

Porto suddenly found themselves attacking that most infrequent of occurrences, an Arsenal side not set up to defend at the optimal level. Declan Rice, so impressive playing on a yellow card over the preceding 91 minutes, was too slow to get up to Galeno. The Brazilian might have missed the two easiest chances of the night but he made amends with the most challenging, bending the ball brilliantly inside David Raya's far post in the fourth minute of added time.

This was Arsenal's junior moment, a young side, many of whom were playing in their first Champions League knockout game, failing to appreciate that it was only halftime in the tie, as the old cliche goes. Better to preserve parity before you get them back on your ground. It may be a new experience for Arsenal but it was not for Mikel Arteta, who doubtless had flashbacks to nine years ago when his teammates frittered away an added-time goal to halve their deficit to Monaco by chasing an equalizer and then conceding a third.

That killed the tie for that team. It is hard to believe that Arteta's, who responded to their festive setbacks in the Premier League by becoming its deadliest attacking force, will not force their way back into the competition next month. He seems to appreciate as well as anyone that his team cannot be this bad again.

"We lacked threat," said the Arsenal manager. "We lacked much more threat, more aggression [was needed], especially when we had the ball in the final third, especially in the back as well, more purpose to help them. We will tweak a few things to attack better, especially because, to be fair, we haven't conceded [chances] at all, but we can do better."

For starters, they could be live to how the game is being managed. Arteta hinted at dissatisfaction with Serdar Gozubuyuk, noting that when set pieces went into the box "everything was a free kick," but he hardly raved at injustices. How could he? The Dutch referee might have been particularly persnickety but he was consistent in the 36 fouls he whistled. If there was contact, play was being stopped. The Gunners seemed to particularly fall foul of this, the 22 they committed the joint-most in the Champions League proper since Dec. 8, 2021. Another referee may well have seen this game differently. Arsenal should have seen it through Gozubuyuk's eyes. Porto certainly did. From the 55th minute onward, they committed just two fouls. Their visitors made 10. They did not read the occasion.

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Many of those fouls, brushes or marginal contact on a blue and white shirt would have been waved away in the Premier League but England's most experienced continental operators swiftly learn that the terms of engagement can differ radically on the continent. Arsenal could feel aggrieved that the officious officiating rather played into Porto's hands. Then again, what might they have expected from a team who lives for the trolling? Any team with Pepe will play the margins as well as their opposition, as they have done on so many occasions in the Dragao, they dragged their opponents into a muddy skirmish.

Not that there was not plenty to admire at the way in which they repelled Arsenal's 64 percent possession. Packing the middle of the pitch deadened the effectiveness of the Leandro Trossard-Kai Havertz tandem while on the flanks Joao Mario and Wendell more than held their own against Martinelli and Saka. Crucially, at least one of Alan Varela and Nico Gonzalez was primed to drop into the backline if a full back got away. Even Arsenal's best moments only seemed to presage more defensive excellence from their hosts. So when Martinelli elegantly flicked the ball away to Pepe, he must have thought he would soon be bearing down on goal. Instead, Varela flew back to snuff out the danger before it had even materialized into a shot.

That combination of cynicism and excellence rather drove Arsenal to distraction. Martinelli was flicking passes to where Trossard might be three minutes after he'd gone off. Jorginho, presumably introduced to add a cool head to the closing stages, lasered a pass into the space between Rice and Saka.

Arsenal were sloppy. Everything we have come to know about Arteta's team would suggest they won't be again in three weeks. Equally, Porto will try every trick in the book -- and a few they find in the restricted section of the library -- to rile them, to turn the Emirates Stadium against their charges. If it works, then these young players will surely learn from it. After all, nights like this happen to even the brightest of young prospects. At least on this occasion, Martinelli and company have a chance to apply the lessons before it is too late.