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.
Yet again the decisions made by Ten Hag had really affected him and people still attack him for that, but let’s start with the facts here was Ten Hag the worst manager at Manchester United? It is certainly No because aside Mourinho post Ferguson era there was none that won more games like Ten Hag, he won trophies each year and made European qualifications in each here. His decision to twerk the players to the ones he wants didn’t go as planned but I wouldn’t call it a mistake it is a manager trying to do it his own way this is what elite managers do, all managers have regretted signing and selling off of players, so Ten Hag should be allowed to rest now on this De gea controversies.
He is at Bayer Leverkusen now and for every one at the club he is the right manager for the job and that’s it. I know very well he will come under very serious pressure just because of what Bayer Leverkusen has accomplished in the last two years, the Fans will be wanting a match of Alonso’s excellence or even surpassing it, but they also might need to understand that even without a new manager this is a whole lot different team from the previous ones they have and they need to give this man some time to build his own team and also build the chemistry between the players. This preseason losses even though bad are better coming now than when the season starts at least he will address the problem now