Agree, not a big problem for Arsenal, but they have to remain alert to their current condition, when it becomes a full defensive match for Porto, it will be hell for Arsenal. I previously forgot to mention that apart from Otavio, they also have Pepe there, so Arsenal must really solve Porto's solid defensive problem. Remember that Porto only needs a draw here to advance to the next round.
Pepe is old his physical endurance must not be as good as when he was young. against a young Arsenal team playing at home and spartan pepe will definitely drain physical and energy so performance for 90 minutes will definitely not be stable. as long as Arsenal does not concede a goal first or the players do not quickly despair, they can definitely advance to the next round.
Everyone also knows that an athlete's performance will decline as they get older, but I think you can rethink your words. Look at the rating on their match yesterday, he played well with Joao Mario, Otavio & Wendell. Usually older players will be played for half game, but not Pepe, he played the full game. Arsenal still have the biggest opportunity to progress to the next round, but that doesn't mean we have to underestimate Porto, we'll see what will happen in the second leg. Honestly, I wouldn't bet on this match because I think it's too risky.