Mikel Arteta has known only frustration at Arsenal in the Champions League last 16. When he was a player, he tasted defeat at this stage of the competition for five consecutive seasons – part of a wider run of seven last‑16 exits from 2011 to 2017. That was then. But here, after a stressful night in Porto against a team with rather more recent experience of this kind of occasion, when the margins were so excruciatingly thin, Arteta got the same sinking feeling. Nothing has yet been decided. And Arsenal will take heart from when they last won a Champions League knockout tie. It was in 2010 against Porto when they overturned a 2‑1 first‑leg deficit from this magnificent arena with a 5-0 victory in the ­Emirates Stadium return. This was another first-leg battle defined by defensive tightness, the closing of spaces, a lack of rhythm; the whistle never seemed to be far from the lips of the referee, Serdar Gozubuyuk. Porto defended deeply. They made life extremely difficult. Even more pronounced was the impression that both sets of players were acutely aware of how much an error might cost. Nobody wanted to make one. It was cagey from the first whistle for a reason. Arsenal would surely have taken a scoreless draw beforehand and the feeling would harden as the minutes ticked down. And yet at the very end of stoppage time, Arteta’s team were unhinged to spark wild scenes from the Porto bench, substitutes and staff ­members streaming everywhere; the crowd in delirium. Arsenal failed to clear once and then again, Gabriel ­Martinelli passing straight to Otavio who returned it to Galeno, 25 yards out and to the left of centre. The danger was not exactly pronounced. But with Declan Rice dropping off, the Porto winger stepped inside and shaped a curler for the far top corner. When David Raya clutched at thin air, Porto had lift‑off. It was Arteta’s first Champions League knockout tie as a manager and a stark contrast to how his team had breezed through the group stage on their return to the competition after a six‑season gap. Arsenal had plenty of the ball and yet it was mainly sterile possession, lacking in zip or incision. They did not have a chance in open play that was worthy of the name, all of their threat coming from set pieces whipped in by Rice. Gabriel went the closest at the start of stoppage time, heading just off target. There was not enough aggression from Arsenal and they paid a heavy price when they failed to manage the very last minute. Porto were 4-5-1 without the ball but Francisco Conceição – son of the manager, Sergio – and Galeno were quick to get up the flanks in possession. Eduardo Pepê tucked in on the right of the midfield. All three were dangerous. The idea was to break quickly – the hosts did not need to hog the ball – and they flickered during a first half of high tension and only one real chance. What a chance it was, the stadium sound guy so convinced that Galeno had scored that he pressed play on the celebration beats before quickly locating pause. Galeno had scuffed the first shot somewhat after a Conceição ball in from the right on 22 minutes, ­watching the effort fly off the far post, but when it ricocheted back to him he had to score. He lashed inches past the other post. Arsenal’s defenders had seemed to freeze. How had the net not swelled? It had been important for ­Arsenal not to do anything silly in the early running; the order of the ­evening was to give the crowd nothing to rally behind. Which only made Rice’s ­yellow card after 67 seconds so ­worrying. He was never going to get there before Galeno and he did not, stretching in to foul him. Rice clambered up on to the tightrope. Arsenal were too predictable before the interval, relying purely on corners. It felt as though Rice took an age before delivering them. A deli­berate tactic to provoke anxiety and chaos inside the six-yard box; physical duels, too? Or just a symptom of ­Arsenal’s lack of purpose? They could not quite profit, William Saliba ­heading the clearest opening off target after winning a wrestling match. Arsenal’s fast first halves have been a feature of their Champions League campaign; 12 of their 16 goals have come before the interval – no other team in the competition have scored more. This occasion was different, Porto rather wilier opponents. They may lag seven points off the domestic title pace – it will be 10 if Sporting win a game in hand – but they have been regulars in the last 16 of Europe’s elite tournament under Conceição. Arteta had stuck with the XI which handed out drubbings to West Ham and Burnley; Kai Havertz in the pocket to the left, Leandro Trossard as a false nine. It was a night, though, when his team could get little going.