I am also surprised that Manchester united was unable to win mediocre team and could not even draw the match but went home defeated. I know that the junior boys were the ones that played the match but it would have being better if United won the match to save good records from now before the new season starts.
However, the result of the match does not show that United is weak because it is just a friendly match and have nothing to do with the main squad performance. I believe that there will be a lot of improvement in the team performance next season and United might finish at top four next season because most strong clubs just changed their coaches.
There's nothing important about pre season matches because the most reasons why teams play friendlies before the league starts is basically to keep fit and prepared against the start of another season so Rosengburg match against Manchester United should not be taken seriously since the match was played by majorly their junior team players so it's nothing to worry about. I strongly believe that since Manchester United were able to make it to the Europa league after winning the F.A cup last season, they will be more prepared and hopeful against the upcoming season so that they can also end in a favourable position so let's just let go of the outcome of the pre season matches till the main league kicks off then we can be able to justify their performance.
I don't know why people are creating much fuzz about this match, I mean if you get to see the line up alone then you would know that the game is not one that should have any impact or should be taken even seriously because I believe it was all the youngster that played that game and besides pre-season games like you said are just for staying fit and nothing more because I have seen teams that have good form during pre-season and then turned to flop when the real competition starts.