Losing Bellingham is a strong blow for Madrid at the moment because he does have the ability to play in different positions while in the squad, but we also cannot deny that the arrival of Mbappe has at least filled the hole that Bellingham has left in the front line, so that currently Ancelotti just need to find the right formula to overcome the problems in the midfield,
Bellingham injury will certainly have a little setback to the real Madrid team because he handles a very crucial area for the club which is midfield and I don't think there is anyone who can handle it more than the way he does because his performance reduces the workload of there defense line since there defense are not that strong, actually with what is happening now on real Madrid is very good they were able to signed Mbappe because a situation like this players like Mbappe and some others will certainly no how they can remedy the challenges they feel they are having.
The little problem they will have just as you said is there midfield because Bellingham plays a vital role on that area, so is left for there coach to develop and use the right strategy to cover up until Bellingham returns from the injury because he may be recovering much quicker than we anticipated.