The performance from united and arsenal are still questionable at this moment. The opening match of EPL has not yet commenced. We don't know how good arsenal and MU right now in the real match.
And still when the premier league starts it'll not determine if Arsenal and Manchester united will be good at the champions league because this are two different league, with different atmosphere. I don't trust both clubs to be able to represent the premier league effectively at the big stages, I would had preferred Liverpool and Chelsea but they were awful last season and lucky Chelsea didn't get eliminated from the premier league talk more of competing in the champions league. It's been long Arsenal played at the champions league so I don't expect them to start off with dominating performance but I wish them the best.
I am not expecting much from Arsenal in the Champions league competition because I know all their focus will be put into the premier league games no doubt about it. The champions league will be more competitive which arsenal might find it difficult to be the favourite team to win, but with the premier league game they have chances of achieving it because arsenal is one of the best team in Premier league as last season, in this season they made lots of good signings that will improve the performance of the team for this season. Arsenal performance in champions league this season is unpredictable.
It seems that CryptopreneurBrainboss judges based on whether he likes a club or not, and I think Manchester United and Arsenal can achieve more success than we imagine. Especially Manchester United is a successful club in the past. I understand now is a different situation, but that doesn't mean we can say that about them. Last season there was also not much Liverpool could do in the Champions League, especially when talking about Chelsea, we all know how they were last season.
So I still have faith in them (Manchester United and Arsenal) on the European stage.