It is true that Al Nasar's quality of play in the Saudi Professional League was very poor.
Al Nassr finished the season as the runner-up. It is strange if you claim Al Nassr performance is very poor.I know Al Nassr failed again to win the title of Saudi league but it is untrue if they are very poor. They won't finish in 2nd place if they are very poor. Anyway, if you take a look the squad quality, Al Nassr has enough top players. As far as I know, Al Nassr is in the 2nd position if we count the market value of the squad.
Cristiano Ronaldo failed to show consistent performance in the Saudi Professional League. Al Nasar's performance was so bad because of him alone.
This is totally non sense! I guess you don't follow the matches on Saudi league. 
If you follow the league, you must be aware that Ronaldo becomes the top scorer this season. He even made a new record, he scored 35 goals in 1 season. As an individual player, Ronaldo performed quite well. He contributed better than the previous season. But Al Nassr still can't be the champion because Al Nassr has bad defense in this season. It is wrong if you blamed Ronaldo's performance.
Source: sportingnews.com
Sure, guy! Next season is Al-Nassr's chance. Despite his strength, Ronaldo can't do it alone. Obviously, defense needs work. Open transfer window. This is their time to get great players that fit the system, lock down the defense, and allow Ronaldo shine up top.
Every season is Al Nassr chance. I hope they won't fail again. 
Sure. If Al Nassr want to be the champion, they need a better defense. Buying some defenders in the next transfer window is the way to improve it. But they must also improve a bit their midfield line, I think it looks not strong enough. In some matches, they failed to dominate the ball possession. It indicates the midfielders are not good enough to control the match.