It was a very painful defeat for Arsenal. They controlled the game but Porto had an excellent defense. Arsenal didn't even have a shot on target. But the match was a painful defeat for Arsenal because Porto was able to score a goal in the 94th minute. I think the match will end in a draw.
But this result still doesn't put Porto in a safe zone because Arsenal is a strong team and a 1 goal lead will only make a small difference. Arsenal will host the second leg so I think Arsenal still has a chance to turn things around.
Arsenal did have a chance to turn things around in the second leg at home when they hosted FC Porto because from the results of their match yesterday it was also very clear that Arsenal were indeed able to control the ball very well even though it was quite difficult to penetrate FC Porto's defense. And I also agree with what you said that a one goal advantage is not enough capital for FC Porto if in the second leg they cannot score another goal against Arsenal.
Porto managed to break most of the predictions, I believe many people lost their bets in that match, I myself didn't expect Porto to play very disciplined defensively and frustrate Arsenal. Using a defensive style and relying on counter attacks made Arsenal overwhelmed, as proven by this strategy being effective, Porto succeeded in winning the match 1-0 on aggregate. Actually, Arsenal played very well, and Arteta squad dominated the match, but unfortunately they failed to maintain concentration in the final minutes.
Arsenal are in a good trend, but they often suffer shocking defeats like this. There is still a chance for Arteta to take his team through to the quarter finals, because Arsenal will act as hosts in the second leg. However, it is possible that Porto will play the old way in the second leg, parking most of their players in the penalty box area to force Arsenal to a draw. Arteta must find ways so that his squad can create more creativity, and balance equally well when attacking and defending.