Yes, Real Madrid doesn't have a commensurate player who can replace Benzema when he can't be used. And it really has an impact on Real Madrid if we look at it last season. I think Ancelotti is more aware of this, he should be. But for now I have not seen which player he has prepared for the position (game-wise).
Benzema is a great champion and Ancelotti a very competent and experienced coach. Binding to only one player is a risk, he can get hurt and other things that can't make him play well. He certainly has a replacement, maybe we don't know.
Knowing Ancelotti who played and coached in Italy, I think there is a plan B.
But with their current condition it seems that this is not wrong I think because apart from the worst possible situation there, Benzema is one of the players who can be relied on for now for Madrid.