It is true that football is not a game of one man, but of eleven. So, for a team to win, a team performance is a must. The addition of one player to the team is not good news for PSG, because they have not yet won a semi-final. Similarly, Ousmane Dembele playing against Arsenal is not bad news for Arsenal. Because Arsenal will have to play against eleven men, maybe something will be different.
The only thing I see in Arsenal's last match against PSG in their first leg is that it is because Arsenal are not at their full squad that their performance was not as well as the people's expectations. If not, I don't think the game will end in the victory of PSG, but nevertheless, let us wait and see how the comeback will go.
PSG is not yet at a stage where their semi-final is guaranteed. They are only 1 goal ahead, which may not be a long time to pay off. It is not possible to say anything until the end of tomorrow's match, because Arsenal have already defeated Real Madrid at home. In other words, playing at PSG's home does not mean that Arsenal will not be able to play well. They are capable of performing at the opponent's home field as well, which they have already proven.
That's just the fact about the tomorrow between Arsenal and PSG. For those that are giving PSG high hope in tomorrow's second leg, I think it's too early to give them that hope, because they were able to defeat Arsenal in the firstg. That doesn't mean that they can do the same thing in the coming match. It will be very difficult for them to make that happen.