I wouldn't say Leverkusen's performance as a team that won the Bundesliga trophy last season is mediocre at the moment. They are still playing well, it's just that their performance is not as good as last season. If Xabi Alonso gets the players he wants in the transfer market later, maybe he will fix his shortcomings this season. Although they are behind Munich in points, they still maintain their position in the standings well. Their chances for the Bundesliga trophy may be getting smaller, but they still have other competitions that they can try. So does Munich.
Yes, the transfer market will provide an opportunity for Alonso to buy some players needed so that Leverkusen can perform well. I am not saying that Leverkusen's performance is bad. But compared to last season, Leverkusen did experience a decline in performance. But they managed to beat Munich in the DFB Pokal which means they have managed to eliminate one of their toughest rivals to win the title. So far, Leverkusen has also performed quite well in the UCL by being in 6th position in the standings which means they will qualify directly for the next round.
I tend to think that it is not Leverkusen who are in decline, maybe other rival clubs have improved, or maybe Xabi Alonso's style of play is no longer as effective as last season so it is difficult for him to maintain the team with the level of consistency of last season. Maybe bringing new faces into the squad can help Leverkusen navigate the Bundesliga better. Let's see in January if Leverkusen has such a plan.