Currently I'm yet to see any lagging aspect of Leverkusen that will need a replacement but then if they proceed to strengthen their bench I think it will be in their best interest just should Incase certain uncertainties comes up they wouldn't be caught unaware, if they have got the financial capacity to strengthen the teams reserve I think it will be a good one so they stand prepared enough against their biggest rivals in the bundesliga.
Bayer has a very impressive winning streak, but it cannot last forever, and it is obvious that in order to break this streak, a serious opponent will be needed, Dortmund is the best suited for this. Bayer is 10 points ahead of Doromund before this match, and if they win, this will not help Dortmund return to the title race much, but it will allow them to enter the top 4, and of course it will give Bayern a chance to return to the top of the standings.