What you said is true. It is the midfield control that he really needs to do something about. kroos is not just any how player, his pattern of play is very unique so it will be hard to replace him. No matter how good Bellingham is, he can't be compared to kroos in terms of how to control ball. The attack is really good with vini ,Mbappe and others, however if the midfield is not strong it will also become a problem. They should get a good replacement . I believe xabi Alonso have a good experience and would definitely get a good replacement.
Toni Kross and Luka Modrich are very skillfull midfielders. Maybe we have not seen these two midfielders from too many goals. However both of these were have the ability to changing the entire match scenario at any time. Moreover , we cannot verify the capability of midfielders by considering the number of goals only. The main responsibility of a midfielder is not to score goals. The main responsibility of the midfielder is to keep the midfield stable, and as a result the attack and defense's performance is strengthened.
Xabi Alonso will not find a worthy replacement for these two midfielders, it is sure. Bellingham is a good midfielder, but his playing style is not similar neither Modrich nor Kroos. Even so, Madrid's midfield has talented midfielders like Bellingham, Camavinga, Diaz and Valverde. If Xabi Alonso's match strategy is good, Madrid's midfield will also be stable.