When Arsenal was drawn against FC Porto in the round of 16 of the UEFA Champions League, I expected Arsenal to win against FC Porto but with the recent performance of Arsenal, I'm not sure if Arsenal could be able to beat FC Porto. If FC Porto can play well in their home, I believe they will beat Arsenal.
It seems like you are not making a prediction for a match, but only stating the possibilities that could happen to the two teams, which in general is still not certain to happen as you say. And if you know how fierce the competition is in the Champions League, then next time you probably won't make a comparison with what Arsenal experienced in their local match with the match that will come next month in the Champions League. Because here all the teams play by giving everything for the sake of winning, so it's possible that FC Porto won't be able to win when they face Arsenal here.