Santos' men are regarded as one of the outsiders to win the showpiece in France, but the 61-year-old believes much will depend on the form of their star man.
"Ronaldo is important for Real Madrid, but he is at least as important for Portugal, if not more," Santos told Marca.
"If you have a player who scores 50 or 60 goals per season, someone who could score from out of nowhere, he is always going to be important. It's impossible to quantify how important he is.
"But not even Cristiano can do it all on his own. Nobody can win games on his own. You are always going to need your team-mates' help. But there is no denying he is hugely important."
Ronaldo was unable to guide Portugal to success at the 2014 World Cup due to fitness issues, but Santos is confident the prolific attacker is fit enough to make an impact in France, despite his injury problems towards the end of the season.
"We are always talking about players having to be 100 per cent, but that's not the reality. There are always exceptions," Santos added.
"Ideally, everybody would be 100 per cent fit, but nobody would leave out Cristiano if he is at 80 per cent. We are keeping a close eye on his fitness.
"It would be different had he been seriously injured. We would not have called him up if we thought he couldn't play."