I agree with you with what you said about Manchester encounter against Nottingham Forest, but one thing you should understand is that, this is football and different games comes with different approaches, so for the fact that Manchester United lost against Nottingham Forest doesn't mean they are not going to do better against Viktoria Plzen in an European competition, because I still see a little bit of zeal on the side of the players and the new coach to mark a difference from where they where coming from. So I'm seeing a win tonight, and also a win in the Manchester Derby, Because City on the other hand are currently in a losing spree, so I believe United shouldn't be left out on catching on, on the free gig City are currently sharing.
United beating Viktoria Plzen makes more sense to me than beating Man City in the Manchester derby after that. Viktoria Plzen and United are level on points now, but I don't see United falling behind in the game. They will be the favorites.
The points will be crucial for them in their bid to progress further in the Europa League. In the Premier League, I think the competition will be much tighter. Amorim still has a lot of work to do to see United perform well.