Erik is a good coach but trying to fix what has already being broken takes a lot of time, not immediate effect. Bayer Leverkuseen made the biggest mistake selling ETH and definitely there would signs soon enough.
There is something that happened internally that the media hasn't gotten the full story on. Eric Ten hag removal from Bayer Leverkusen is partly because of the poor performance they have had recently but for a team that lost a good number of their first eleven I don't think much was expected from them.
The stories aren't conclusive but there have been suggestions that Eric ten hag was having issues with staffs of the club and with his relationship with some players when he was at Manchester United it is easily believable.