Yea he was actually a good manager at Ajas Amsterdam before getting the job at old Trafford meaning a manager might be good at a particular but flop in another one although you have stated already that Netherland league can not compared with English premier league and I believe that such may be a contributing factor why it wasn't easy for Eric Ten Hag in the English side Manchester United having said that, the players he had in the club during his stay in Manchester United could also be the reason why he couldn't get it right there, I have Come to understand that Eric Ten Hag alone was not the only problem Manchester had during his managerial role in the club, the palyers weren't the best themselves, although I won't conclude if he will be the best in Leverkusen but lets see how events unfold although he might prove himself to be worthy of the role he's been hired for so that he wouldn't be seen as a failure but the players has adapt to his coaching style for thing to work perfectly as they want it to be.