If utd had won every game and Mount scored and Rashford was playing well, Garnacho was tearing it up, I think there would be numerous posts of how the team look like challenging for the title. Fans are very fickle like that, lose a few and everyone plays down the importance.
The truth is pre season isn't like it was years ago, players have fitness programs which they stick to and the science involved means players don't lose their fitness as in previous eras.
What pre season is used for is sharpness, working on patterns of play, maybe experimenting with one or two in different roles, maybe a change in formation. You can be assured though, the team are being prepared to treat these games as prep for the first league game of the season.
If the team are struggling then it's a worry for sure. Not the end of the world, but winning breeds a winning habit, but so too does losing. If Utd lose the next game then the manager is going to get pretty concerned at how you get back to winning ways. You could see his annoyance in the game yesterday, if players stop doing what they're instructed too, if attitude is poor, this can manifest and others follow. There are no friendlies in football anymore.