It is an embarrassing result for Ten Hag and his team for allowing themselves to be trashed by Flamengo U20 team, this is a youth team we are talking about. It might not be an important game but Such results are not good for the team morale especially before the season kicks off. It is obvious they need to improve and do better next season, Ten Hag also needs to prove Manchester United wrong for sacking him by getting good results next season.
I actually do not see it as embarrassing enough, I even feel it has some intentionality in it, maybe they used that to get the team ready for the season and they actually did not take that game serious enough to want to win, in as much as I will not want to judge this game, I still think Ten Hag has to improve with his tactics and his technicalities so that by the actual season he will be able to get the team in their best form and get team in their best form to be able to deliver the kind of results and possibly win trophies as expected of them.
I don't see no intentionality in losing a game to Flamengo U-21 regardless of the fact that it wasn't a competitive game. Not taking the game serious doesn't mean that they should lose to an inexperienced team but if Erik Ten Hag in reality didn't want to win the game, then I think he's not mentally ready to have a good debut campaign with the former German champions. Xabi Alonso during his spell at Bayer Leverkusen have set the stage high for any manager who'll succeed him and if Ten Hag fails to match or better his records, he'll likely be seen as a failure which is why I think he should do anything possible to help the team improve especially in their mentality.
Winning friendly games might not count as a significant result but it sure has a way of improving the winning mentality of the players so it's important to get results in such games especially for a manager that just arrived a new club.