PSV is not a favorite against Arsenal, especially if Arsenal plays at home. Arsenal have performed well at the start of this season even though they have also struggled to win their last 4 matches in the Premier League. This will be a good match for both of them considering their meeting record in the last 4 Champions League matches. Arsenal and PSV both have 1 win and 2 draws and 1 loss each, so this will be a tough match for both.
In the champions league; there are no favorites in the group stage; anyone can beat anyone, anyone can defeat anyone. Also in the champions league; there is nothing like league form, you can be on a 20 game winning streak in your league; and get defeated in the champions league. Arsenal needs a top performance to compete in the champions league.