The former A.C. Milan coach replaced Luis Castro last month and began with seven wins, before losing to Al Taawon in the Saudi Cup where the usually prolific Ronaldo missed a penalty in additional time.
"Of course, I have always thought that Cristiano is a great champion," Pioli told the Saudi Pro League's media service on Wednesday.
"But above all, a great professional with a great work ethic, a great culture of always trying to express oneself to the maximum.
"And therefore, I am discovering a boy who is no longer young, but with the same enthusiasm, with the same passion.
"As when he was a kid, he wanted to do things in wanting to always try to give the best, to reach the maximum, to score many goals and help the team. So, definitely a pleasant surprise".
The Italian said that he was very happy to be approached by Al Nassr as he wanted a new experience.
"The fact that there is Cristiano in the team was also an important thing and so I accepted very willingly with a lot of enthusiasm, with a great desire to be able to do well," he said.
Al Nassr, third with 18 points, face league leaders Al Hilal on Friday in the Riyadh derby.