Against Croatia, the history is on Portugal's side - as it has won all three of their previous encounters without conceding a goal, including a 3-0 victory in the group stage of Euro 1996. More recently, Ronaldo scored the only goal in a friendly win at Geneva three years ago.
"This is a totally different match. I warned my players because we will play a final tomorrow," Santos said. "Croatia has a great team. (Luka) Modric is arguably one of the best players in Europe right now, no one doubts that."
Santos took over from Paulo Bento in October 2014 after Portugal's disappointing showing at the 2014 World Cup and poor start into Euro 2016 qualifying with a 1-0 defeat at home against Albania.
After leading Greece to the knockout stage of Euro 2012 and the 2014 World Cup based on a tightly organized defense, Santos followed the same philosophy and started rebuilding the Portuguese team.
One of the new names was Sporting midfielder Joao Mario, who made his debut as he replaced Ronaldo in Portugal's first match under Santos, a friendly against France at Saint-Denis. Mario has featured in all three matches Portugal played so far at Euro 2016.
"We are aware of what we've done so far. We were very good in some aspects but we need to improve in others," Mario said through an interpreter. "The group stage is behind us now, everything changes. We want to do much better than before."
Mario said "it was not ideal" that Portugal had just three days to rest following its final group match, compared to Croatia's four.
"We know we have not enough time to recover," he said. "But if you get to the knockout stage, you always feel energized."