You are right, if Arsenal can get at least a draw in their first meeting then their chances become bigger, because in the second meeting they will play the match with home status, of course it is a very good opportunity for them.
But Porto is not a club that can be conquered easily, their presence in this round shows that they can indeed compete. In the group stage they also did not disappoint because they could collect 12 points from the 6 matches played, actually it was the same points that Barcelona had, but they lost in goal productivity so that only put them in 2nd place.
Arsenal are also in an unhappy state for sure, as they failed to win in their last match in the domestic league and kept them in 2nd place.
Arsenal is currently really unstable and I think it's really going to affect how well they will be performing in the forth coming champions league fixture their recent loss aswell will also affect their morale as their confidence may be shaking because they would be surprised they lost to such a weak team.
But then in the UCL I'm expecting they will be able to put up a better performance because they are awareness bof the competition in the champion's league and the pressure is such that every match counts and it's an opportunity to either determine how long you are going to stay in the league or if you will be leaving anytime soon because each time you drop points you chances of proceeding further in the UCL reduces. But then I'm optimistic about Arsenal's performance in the UCL this season.