Even though Darmstadt 98 is not a team that Bayern Leverkusen needs to worry about, they should not take this match lightly. Because these three points are important, apart from keeping their distance from Bayer Munich, they will also extend Leverkusen's record of never being beaten in the Bundesliga.
I think Xabi is also aware of this, he has to be smart in rotating players to avoid fatigue and of course injury. The reason is that after facing Darmstadt 98, Leverkusen will face VfB in the DFB cup and then face Bayern Munich. If you look at the next few matches, it looks like Xabi will also rotate players when facing Darmstadt 98. Because the match against Bayern Munich really determines which team will stay in first place until the end of the season and win the Bundesliga title.
After having unusual draw against Gladbach they will not take Darmstadt as easy even their record is worst in this season, and they are conceding 5 goals in every 2 games which are making them mockery with no win in last ten games and most chances they could be back into Bundesliga 2 in next season with Mainz which is also at their level, but now Bayer Leverkusen needs to have some strong comeback after frustrating performance in last game because if they will be not gone strongly then surely things could be not favourable with just 2 points difference left, and they need to be at their usual strategy.
Rotating players is good strategy for the every coach because with these players can take good advantage and reserve players can also make some good contribution which make them prepare for the big games while they are needed due to any problem or injury Xabi Alonso is surely currently doing his best which is making things easier for them in all competitions.