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.
True, Real Madrid have a stable defense even that is confirmed by the appearance of their excellent goalkeeper, Courtois. Maybe some people think that Real Madrid's defense is weak because they saw Courtois being the last stronghold that was difficult to penetrate, so Real Madrid's defense didn't seem so dominant.