Ronaldo, Portugal's all-time leading scorer, missed a penalty in the 0-0 draw with Austria in Group F on Saturday and has failed to hit the target in four of his last five penalties for Portugal and Real Madrid.
"If there is a penalty in the next game, Cristiano Ronaldo will take it and he will score," Santos told reporters. "He is used to scoring goals, he is a winner and he also reacts well when he makes a mistake. That is what we expect from him."
Ronaldo, who won a record 128th cap on Saturday, has had a disappointing Euro 2016 so far, failing to convert any of several good chances as Portugal have drawn both of their games.
Left-back Raphael Guerreiro has increased the pressure on Ronaldo's place as the free kick specialist after scoring with a superbly-taken effort in a 3-0 friendly win over Norway before the tournament when Ronaldo was not playing.
"Cristiano always wants to take the free kicks and it has always been him who has taken them," said Santos. "If Raphael had not scored that goal, nobody would be talking about this."
"We played really well against Austria without scoring and we have to continue with this attitude. We made Austria, who are ranked 10th in the world, look like a small team."
Santos said Portugal should not be ashamed of "being ugly" and that he had told his family that he would not be coming home until the day after the Euro 2016 final when he expected a party.
"Sometimes, I get the feeling that we are ashamed about being ugly, or not being pretty," he said. "If we have to be ugly and that brings us efficiency, we have to be ugly. I'm ugly by nature and I don't mind being even uglier."
Portugal need to beat Hungary on Wednesday to make sure of qualifying for the knockout stages but Santos said he would be staying in France until the final on July 10.
"I am only going back to Portugal on July 11 and I am going to be welcomed with a party," he said.
(Writing by Brian Homewood. Editing by Adrian Warner)
