I don't think Real Madrid's defense is weak last season. Their defense is stable enough. At the same time they have a great goalkeeper. Courtois is very talented. Real Madrid should strengthen their attack.
If someone says Real Madrid defense is weak, then what is the most losing goals ever conceded to Real Madrid, Real Madrid have Courtois the greatest goalkeeper and even though they escape the Real Madrid defense but will not have a chance to score a goal from goalkeeper Courtois.