What Xabi needs to do is turn star players into team players. Managing star players is very difficult. Because they want individual success, they want to attack every position themselves. However, having a unified team is beneficial to the team. That's why Real Madrid didn't win the championship last year, despite having so many star players. Mbappé and the other players need to focus entirely on teamwork. Xabi has to orchestrate this.
Xabi Alonso was effectual with Leverkusen. They won Bundesliga without losing a single game. This is absolutely Xabi Alonso's endowment. you know their performance was superb even though there was no star player. They were not resting on a specific player, as a team they tried to play adroitly and won. Madrid's condition is opposed. Los Blancos have a lot of star players, they also want solitary success. Xabi has to change this mindset, in particular I would say Mbappe has to change his style of play. It is true that he is capable of scoring a lot. But in addition to scoring, he has to have the mentality of helping his teammates. If Madrid fails to play well as a team, then they will not be able to win the title next season either.
And this is the problem Xabi Alonso is currently evaluating: managing the egos of star players. The defeat to PSG in the Club World Cup opened Xabi's eyes to the problem Real Madrid currently face, their inability to work together as a team. Vini and Mbappé often appear selfish when played together.
Xabi will certainly face a unique challenge in coaching a team full of stars. The coach must be clever in managing the egos of star players so they can play as a collective. This is very different from managing Leverkusen. No single player stands out, the team must play as a team to win.
If Xabi can't address this issue at Real Madrid, it will be a problem for Real Madrid going forward, especially in the Champions League, where they will face tough opponents.