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.
Leverkusen are in a precarious situation at the start of the season, so it's a bit difficult for them to do well this season. I'm still not sure what they'll do now that Ten Hag has been sacked and whether they'll hire a new coach or not. The impact of such a precarious situation will definitely be felt on the pitch and I expect it to be negative.