The way Al Nassr lost, was a humiliating one after spending that much on players. They were clearly the better team, and yet they still failed to win. That is the problem they have, even when they are the better team, they lose, and they can't afford to do that much. Ronaldo lost the chance to get one more cup, and it is really not looking good, been years since he got one, they got one on his first year and nothing after that.
Al Nassr coach is too focused on improving their front line that he hasn't thought about recruiting quality defenders to improve their performance in all areas.
Yes, you are right. Al Nasr's defense has become very weak. If Al Nasr's defense is not changed very soon, then maybe after a while they will have to face a very bad situation. If the defense is not strong, it is never possible to win on the field, just like the defense needs to be very strong to stop the opponent. So Al Nasr's defense needs to be fixed very soon. If they are in such a situation, then they will not be able to do anything very well, they will not be successful in stopping the opponent.