How does a team like Arsenal, not even get a shot on target? That's very surprising. Arsenal had the major share of possession but couldn't get even a single shot on goal. While Porto were considerably efficient in the attempts they did get. Porto had more shots, and shorts on target even though they saw the ball, less time than Arsenal. If Porto can achieve similar performance and shut Arsenal's offence down, they'll go through to the next round.
They just dominated the game without having good enough chances to shoot, that's the reason why they didn't have even 1 shot on goal out of the 7 shot attempts they had. Otavio played an important role in Arsenal's failure to create good opportunities, you can see how he performed in the match, he even got the highest rating and MVP in this match.
The result of this match didn't really surprise me because Porto played at home, there was still the second leg remaining and the match would take place at the Emirates, the only thing Arsenal had to think about was how they would break down Porto's defense if they still wanted to progress to the next round.