Yea, he is more concerned about Real Madrid's defense. If you observed last season, you would see that Madrid's defense was very porous which made it easy for their opponents to penetrate. I believe that Xabi Alonso knows what he is doing and will definitely work on Madrid's midfield too. Different coach with different game pattern, what matters is to win trophies.
This is true, last season Real Madrid has a poor defense. Barca, Manchester City exploited that defense which resulted to their elimination in the UCL and the loss to Barca in 4 matches they encountered each other. Any team that has a strong attack last season will surely score up to 3 goals against Madrid, despite the fact that they kept on scoring too as they have one of the best attack in football currently. However, it doesn't matter, if you keep scoring while they keep on scoring you, the goals are useless. For a team to win, they have to secure the goals they score which is Xabi Alonso's priority this season.