I don’t think Ten Hag will be able to seriously challenge for the Bundesliga title next season. Alonso showed this year that when Bayern worked with Kompany, they became much stronger, meaning Bayern overcame their crisis and competing with a team like that has become extremely difficult. After Alonso’s departure, the situation could get even tougher. Plus we will need to see how well they manage to keep their key players and how Ten Hag handles the transfer market. He is a good coach and he built a strong reputation before his time in Manchester, but I am not sure that is enough for Leverkusen to win the title again.
Not only you and probably most people doubt Ten Hag because he had a failure at Manchester United but we need to trust him and hope he can do much better. Kompany also did not prove to be a great coach when he came to Bayern Munich but he was able to give his best slowly. What made Kompany who he is now is because he has the support and quality players in the team. This is what Leverkusen's management needs to do so that Ten Hag can also give his best for the club's success. If he was able to achieve success as Ajax coach why not give him the chance to achieve success at Leverkusen.
Food point about trust and giving managers time, but I can't help but think that the situation at Leverkusen is far more dangerous than what Kompany is dealing with at Bayern. Kompany took over Bayern with a big checkbook, a history of winning, and a structure that can absorbs shocks. Ten Hag takes over a Leverkusen that lost its brains (Alonso) and its most creative players (Wirtz and Frimpong). Wirtz got offers of €130–150 million on his own, and the whole tactical system was made for a 3–4–2–1, which isn't even his style
If the board supports him with big signings and doesn't get too upset over a few losses, then yes, he may pull off an Ajax 2019. But Leverkusen's approach is sustainable seller, not a galacticos buyer. In the past, they would cash out as soon as they reached their peak. See what happened after their past golden generations (Ballack, Ze Roberto era, etc). The stats show a predicted drop to 69 points, and key rivals already handed them losses in simulated results. This isn't like the NBA's "trust the process". It hopes that Ten Hag can quickly develop a Champions League team with injury-prone players and a salary cap that makes Bayern laugh
Is it possible? Of course, but he has to accomplish more with less. He built on a winning academy pipeline at Ajax. At Leverkusen, he is inheriting the mother of all “afterparties”. If he does it, it will be more than simply calm hands; it will be miracle worker stuff. Let's find out what the summer window actually provides