Leverkusen sold their best player in the Summer.
they didn't, but they did sell a really good player, and reinvested all the money in other very good players that improved the team.
A lot is made of Xabi taking over a 17th place team last year, but that was a bit deceptive, as that team was also the 3rd or 4th best squad in the league. It was a perfect opportunity, take over a really talented group, that was underperforming horribly.
Not to say he’s not doing impressively, of course he is, but sometimes there is more to the story than just looking at league positions!