Santos' men take on a Croatia side that has impressed many at the Stade Bollaert-Delelis in Lens for a place in the quarter-finals on Saturday.
But Santos is adamant those matches are now behind them, saying at a media conference: "This is a final. You can't compare the group stage with these matches, it's apples and oranges.
"We wanted to finish first but after a while, it became essential just to make it through. Now we have to win, and that's what we want to do. We're facing a very strong team and they want the same thing Portugal wants."
Portugal have struggled to break down the likes of Iceland and Austria, however, they may have more space to operate in against a Croatia team that plays an expansive style of football.
Croatia topped Group D, helping Portugal avoid a meeting with Iberian rivals Spain, yet Santos was quick to reject talk that they now have an easier tie.
He added: "How could it be easier to play against Croatia? They're an amazing team. I realise they had an amazing game against Spain but we're very strong.
"Both teams are self-confident. I believe Portugal will win, I'm sure the Croatia coach thinks he will win."
And Santos does not believe Luka Modric's likely return after sitting out the win over Spain will influence Croatia's approach.
"One of the best games in the Euros was Croatia against Spain, and Modric didn't play, which tells you about the Croatia team," Santos said.
"He's one of the best players in the world, nobody doubts that, but I don't think they play differently with or without him."