Ten Hag is still adapting, and the preseason results shouldn't be taken too seriously, but they shouldn't have lost 5-1 to a mediocre club. Because of Ten Hag previous poor managerial record with Manchester United, people tend to immediately judge him with harsh criticism. In my opinion, Ten Hag still needs to experiment a lot to build a solid team, as Xabi Alonso did before him.
It's normal for a new manager to adapt to a new team and a new league. But as you said, Ten Hag had a really bad track record at Manchester United. He also couldn't handle press conferences, and I often saw clips of Ten Hag blaming others or certain circumstances for the team's losses. He also made some controversial decisions, like releasing De Gea and buying Onana. Yes, his reputation at Manchester United was really bad. But on the other hand, some people said it wasn't entirely Ten Hag's fault, as United's form was also declining. But the defeat to the U-21 team will certainly raise a lot of doubts among fans. They will try to compare Ten Hag and Alonso in the future.