Manchester United beating Bayern Munich in Germany is not a totally impossible in the world of football. Why th person made such a statement is that the current form of Manchester United is not something that is encouraging. If Manchester United should be on their prime, there is no club they cannot beat, be it home or away. In the other hand Bayern Munich is not even that club we used to know some seasons ago. They are strategically trying to come back to their old form, which they have not attend to currently.
Manchester United were underrated because of the poor performance they keep recording in EPL. Who would have thought the Red Devils will net three goals against the hyped Bavarians, absolutely nobody. There was a chance for an equalizer but manchester united goal keeper, Andre Onana committed blunders that made them concede unexpected goals in the game. Perhaps Onana acknowledged it and would look for more exploring options to triggered, they're targeting best results this season, and they will definitely get prepared for it.
I think they are not being underestimated in the Champions League this season, especially since they have just returned to the Champions League, of course the spirit of Manchester United players is quite high. It's just that Bayern Munich experienced a decline in performance in the second half which allowed Manchester United to score 3 goals.
Onana is not very consistent, he often moves in situations he shouldn't. I think he should be able to control his instinct level which is quicker to catch the ball rather than continuing to make blunders.
However, the final result was in favor of the hosts where they were able to get 3 points at home which made them temporary leaders in Group A.