@pusaka if Leverkusen want to stay in the title race then they will need to dip in the January transfer window and buy one or two more player’s to further strengthen their squad. However if they decide against buying then Bayern will have a clear advantage because their squad is really strong and I don’t think that Leverkusen can stretch the title race till the very end with their current squad.
The better thing for leverkusen tries to keep their players. I have seen some news said that if other clubs have been targeting leverkusen's players to be recruited by all of them in january. This will make leverkusen to lose its strength. It's not about buying the new players.
It's about how leverkusen will try to defend its players that have been fully adapting to the club. This is the most difficult part. Missing one or two important players will be obviously affect the performance from the club.
Leverkusen needs to put it as its priority rather than try to buy the new player. There is no guarantee that a new player will be able to perform at the same level as the existing players.