Cristiano Ronaldo was also in a very good form last season as he scored the most number of goals in the Saudi Pro League but it didn't stop his team from losing the league title to Al Hilal. The problem of Al Nassr isn't Cristiano Ronaldo and the earlier they start winning games especially against big teams like Al Hilal that they failed to beat last season the better for them in their quest to win a major title this season
I am not saying that Ronaldo is their problem. But I think that Al Hilal has a far more balanced team. Their defense was by far the best and regardless of Ronaldo scoring so much for Al Nassr, Al Hilal still scored one goal more than Al Nassr in total. They had about the same strength in attack, but Al Hilal conceded about half as many goals as Al Nassr. That is why bringing in great strikers is all nice, but the balance was neglected. Al Hilal may not have a Ronaldo in their attacking line, but they have several good defenders.
But even so I tend to agree with the perspective from @tiCeR which is indeed a team is required to have balance in every line of their squad to win the championship trophy, al nassr has good support in the front line in scoring goals with Ronaldo, Talisca and also Mane, but when they lead in a match, precisly the defense often a weakness which finally makes them have to throw away that the victory, But now slowly Pioli may be able to balance the Al Nassr squad, because after all, the strategy of the coach is also an important role for a team to achieve and maintain victory.