In my opinion, even though Erik Ten Hag wanted the Bayer Leverkusen key players that left the club in the summer transfer window to remaining at Bayer Leverkusen this season that wouldn't have happened because it seems it was the Idea of Bayer Leverkusen management to cash in on those players. With the attitude Bayer Leverkusen showed in the two matches they played this season, they might likely not make it to the top four.
This same incident happened with Monaco and Ajax after a one season splendid performance and they sold their key players bagging in loads of cash and suffering the season after that. I dont any coach at this point can be blamed for the club's downfall as major players that contributed to the team's previous victory has being sold leaving the club in peril.
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.
Xabi Alonso leaving Bayer leverkusen has really affected them and couple with the fact that alot of their big stars too have left the club so it will take some time before any coach will get things all working with that club again. The best thing to hold onto is the fact that they have achieved alot and with that they can just try to settle and accumulate players so that they can get things all working again so they can meet up with the standard already set in the club.