Perhaps Xabi also wants to strengthen his bench team. As a result, Leverkusen will have a viable replacement even if a player is injured. And their overall performance will not deteriorate. But I doubt if the Leverkusen management will raise big funds in the winter transfer window. Usually Bundesliga teams don't spend a lot of money building squads.
Strengthening the bench team will certainly add depth to the squad which will be good capital to face many competitions in the future. If Leverkusen wants to get good results in the Champions League and become champions again in the Bundesliga then strengthening the reserve players is something that Xabi must do. But of course, as you said, this means that in the transfer market Leverkusen will have to spend quite a lot of money because it will be quite difficult to find great players who want to be bench players.