It is very hard to predict what is going to happen when both of the teams are performing inconsistently.
They overachieved last season when they weren't ready for such achievement and they were able to do this because other top clubs weren't in their best form. The champion league pressure and their last season success is what is affecting Newcastle. Assuming they didn't qualify for the champion league then they won't have had too much pressure on them to also replicate same success this season. Newcastle stats in their last 5 games are very poor, they have been able to only win twice and losing three straight games which is very unlike Newcastle united then there's Ac Milan that has won 4 games from their last 5 games and will also be playing at home which should give them more confidence to win the game.
In Manchester united best form, Bayern Munich would had still win them so Manchester united not been in form isn't an excuse for then to play baldy against them. Bayern Munich are looking favorite for me to win the title this season because of the squad they have, only Manchester city squad do I think can match Bayern Munich squad man to man. The addition of Harry Kane to the team was the final piece that they were missing, Bayern Munich will also be playing at home and that'll give them more advantage. Harry Kane will also be playing against a former league clubs that he's used to their style of play and Manchester United doesn't have the best defense currently which just make things worst for Manchester united.