He succeeded with Ajax Amsterdam before moving to Manchester United, so at a smaller league than Premier League and work for a smaller club than Manchester United, I think Ten Hag will perform well enough and won't get terrible management for Leverkusen. He has to help Leverkusen going through this summer transfer season as best as possible because it is important part of the club preparation for a next season.
Yeah, he actually was a successful coach at Ajax and that was one of the main criteria that Manchester United sort after and he was appointed. But his pattern of discipline was something else which made some players furious with. Had it been he was delivering and was that strict at least it would have made more sense but in this case, he was having a bad record with the club.
If Leverkusen seeks for his services then he has a lot of work to do considering that Alonso who's leaving is actually leaving a big void to be filled by whom ever is going to takeover the club's management.