I think he doesn't underestimate SPL, he is just unlucky because he more often gets injury.
You must understand that it is difficult to play in the best form if a player often gets injury. Neymar and Lewandowski are the examples of this case, they are difficult to play in their best level anymore because of the injury problem.
The best player in SPL isn't only Ronaldo, you must consider other players as well. Mitrovic, Talisca, Mane, Milinkovic-savic are also best players in SPL this season. I also think there many other European players who play well this season. That's why SPL becomes very interesting this season, it is because we have many good top players.
Every player who has experienced an injury will definitely find it a little difficult to return to their best performance because they are a little worried that the injury will happen again if they are not careful. A history of injury is a player main problem because it affects his career, while Ronaldo is still fine joining Al Nassr. So it is natural that he is still quite good at playing in the Saudi League and other players like the ones you mentioned are no less good than Ronaldo.