Ten Hag have had experience in this league before so there's chances that he could do well, I'm not going to conclude until I see how Leverkusen would perform in their first 5-10 games until then Dortmund are the only team I feel can challenge Bayern Munich even though I doubt if any team would take away their dominance like Leverkusen did when Alonso was there. Well with the squad Xabi left for Ten Hag getting into the Champions league shouldn't be a big deal.
It was a very long time ago when Ten Hag managed Bayern Munich II. The current condition of Bundesliga is no longer the same as in the past. Ten Hag must have no much insight about the domestic competition in German recently. I'm sure he needs time to adapt with the local competition in German. Moreover, he is managing Leverkusen who has a big pressure. Ten Hag is expected to strengthen Leverkusen and stop the domination of Bayern Munich. Since some key players have left the team, I think we can't assume it is still the same as in the era of Xabi Alonso. You must be aware that Jonathan Tah, Jeremie Frimpong, and Florian Wirtz already moved to other teams. I personally assume this is a very big deal. Moreover Ten Hag still can't get the replacement with the same level.