I don't think so. Leverkusen sold two of their best players. Even so, the bonding between the players in the squad is not weak. Leverkusen still has a stable squad.
So qualifying for the Champions League should not be difficult for Leverkusen. Teams like Dortmund and Leipzig are not consistent in their performances. We see these teams performing irregularly almost every season.
Its not only about the key and best players that left Leverkusen , the most will be the change of the coach.
For now i would say that it will be very hard the coming season for Leverkusen.
Dortmund and fingers crossed here hopefully will be better in the season.
Leipzig is behind the performance they can do.
You are right. The change of coach will definitely have a big impact on Leverkusen's performance. However, if Leverkusen management had not sold talented players in the squad , then perhaps their overall performance would have been somewhat stable. However, Leverkusen is not a rich club. So they will have to rely on the money received from player sales to run the club well.
It is not possible to predict how dortmund , Frankfurt and leipzig will perform next season. Last season we saw that Frankfurt was able to perform quite well. The team was in third place in the points table. On the other hand , leipzig could not qualify for any Champions League or Europa League platform. Their only target will be to do well in the Bundesliga. This team will also fight hard to stay in the top four of the table.