I also think Borussia Dortmund will be better than Leverkusen this year. For some reason, many people aren't expecting a great performance from Bayer Leverkusen this year. Perhaps this will ease the pressure on the manager.
In addition to the many players leaving, Ten Hag's history with Man United seems to be a strong consideration. Ten Hag's poor history with Man United is still a clear basis for consideration of how Ten Hag will fare with Leverkusen. Although the league is different and the pace is slower, there is still a chance for Ten Hag to make Leverkusen great again. But let's just look at the present. The results of the matches this season will be the best proof of his ability to manage the team.