Yep, I thought Leverkusen use their Academy players, but when I check their lineup in that match, most of them are senior players
https://www.besoccer.com/match/flamengo-u20-juvenil/bayer-leverkusen/202612247/lineupsI think there's no excuse, even though Leverkusen lost Tah, Frimpong and Wirtz, but the current players should able to beat mediocre team. Unfortunately they lost with big margin, this show if Ten Hag was the problem.
You are right, the intensity of the opponents is not that much because they played against a small team, they should have won the match, but the defeat was disappointing to their fans because they expected a better performance from Ten Hag as an experienced coach.
However, I am still considering it as an adapting period for the manager, he need to take time with the players to get to know them better, that is why I am taking that as the only excuse for the defeat, and I think Ten Hag need to also be very serious because Manchester United fans are still saying that he was the cause of their problem, therefore, he need to prove himself.
Furthermore, the management need to help the team by controlling the affairs of the players, they should do their best to convince Granit Xhaka to stay with the club because he is a leader, losing him will affect the team’s performance next season.