How important playing against Alaves compared to facing Arsenal three days after playing tomorrow. The risk will be for him not starting all key players, it is not compulsory that you win some relevant games with key players only, there are still better players aside from the usual first eleven who can help the club beat Alaves tomorrow. I know some key players will begin, but we will see some stay resting to face Arsenal by wednesday. Real Madrid needs their full squad in action against Arsenal, they need to subdue the threat by equalizing three goals within minutes in 90 minutes against a club with the strongest defense.
Both are much more important because if Real Madrid lose, the competition for the La Liga title will be even more difficult for Real Madrid, especially in the other match Barcelona won the match against Leganes. Rotation can be done if Ancelloti is really sure he can win the match even though he only plays a few reserve players because the risk is big if Real Madrid loses again.
Facing Arsenal is indeed a priority because Real Madrid need full strength to turn things around and for me these two matches are quite important that Real Madrid must win.