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.
Rather than a score of 2-1 or 3-2, a score of 1-0 is better for the home team in the 1st leg and is not detrimental for the away team in the 1st leg.
Arsenal's chance to respond in the second leg at the Emirates Stadium to ensure they qualify for the next round is quite big as long as Arsenal are able to play and take advantage of the slightest gap to score a goal.
Porto after getting a 1-0 result in leg 1, they will try not to lose by a goal margin of more than one and try to score a goal at the Emirates later.
For Arsenal, winning by more than 2 goals is a target they must achieve. If Porto in the second leg plays defensively and only uses counter attacks, this will endanger Arsenal.