Xabi has just taken charge of the team. He played one official match at the CWC. Furthermore, some of the players who played there won't be with the team next year, such as Luka Modrić. Therefore, the Spanish manager will need to analyze the team thoroughly and manage Real Madrid in the best way possible to achieve success in the new year. I'm one of those who think Real Madrid will be successful next year. They can be ahead of Barcelona, but the players need to play with each other and build bonds, and of course, they won't have to waste too many individual chances like last year.
Xabi Alonso still has a lot of time to develop the Real Madrid team and from what happened in the Club World Cup I don't think it can be used as a reference because Xabi Alonso still needs time to adapt and also Xabi Alonso has proven that he is a quality coach when handling Leverkusen so the same thing is also possible for him to repeat again with Real Madrid.
I also think Xabi could have a successful career with Real Madrid, because despite a very difficult period at Leverkusen, they didn't suffer a single Bundesliga defeat in the season they won the championship. Even taking into account European competitions, their only defeat was against Atalanta in the final. Now, with a team like Real Madrid, and with the resources they possess, they can achieve even greater success. As long as they compete with a systematic and success-hungry squad like Leverkusen's, I believe Xabi will serve Real Madrid for many years.