I don't think it was an unlucky day for arsenal. because arsenal was unable to make a shoot on goal on target. it shows that porto's defense is strong enough so that none of arsenal's shots or attacks lead to the goal or on target. of arsenal's 7x shot attempts none were on target.
If a team conceded in the 90+ minute then we call it bad luck because they only need a few minutes to end the game without defeat but the fact is they conceded and lost in that last minute their struggle is in vain for 90 minutes and we know it will disappoint. Even though we call it unlucky day, it doesn't mean Arsenal lost the opportunity yes we know that in the next match Arsenal will avenge this defeat certainly not difficult for Arsenal to score 2 goals right? so Arsenal only need to focus on the coming second leg.