It's just a preseason match, and it's still too early to assess how well the team can play under Ten Hag. I think Ten Hag also needs some time to evaluate the players' form. After all, they had a long break during the off-season, and the team lost several players. But it's only a preseason match, and maybe he just wanted to see what each player is capable of. Of course, I agree that losing even a preseason game with such a score isn't a good sign, but it's still too early to judge their form based on one match.
While it is only a "preseason" game, we have to realize, they played against u20, that means that was still a bad game even though it was a preseason game. No matter how unimportant friendlies are, you get to see how the manger is working and what he is trying to do. I get that it won't be as good as they used to be and that is understand with players they lost, and I understand it was just a friendly game.
But two things are seen from this period, one is the fact that they did worse than expected even for a friendly game, and two is that ETH is back to his old self and complaining about taking over a bad team (because of players that left). I believe with these two information at hand, we can expect a 8th place finish.