Questions have been raised as to whether Juve may have been misguided in paying €112million to sign him from Real Madrid.
Santos, though, offered reassuring words to the superstar forward, telling a news conference: "Wait a little and you will see the goals that Ronaldo will score."
The 33-year-old will miss Portugal's Nations League opener against Italy on Monday after opting to skip the international break.
9 - Cristiano Ronaldo is the player with the biggest difference between shots on target and goals in the top five European Leagues (nine). Unlucky.— OptaPaolo (@OptaPaolo) September 4, 2018
Ronaldo's absence was keenly felt during the recent friendly draw with Croatia as Andre Silva failed to impress with a chance to lead the line.
Santos remains certain Portugal still have enough quality to mix it with an Azzurri side that was held 1-1 by Poland on Friday.
"The players have great confidence and great ability, but we also know the quality of Italy," Santos said.
"We know they have a great desire to avenge missing the World Cup and want to show their value in this new competition.
"It will be a clash of two great teams. There are no favourites."
Borussia Dortmund left-back Raphael Guerreiro will also sit out against Italy after picking up a thigh problem.