The Real Madrid attacker, under an injury cloud, has returned to training with Paulo Bento's squad and could play a part in a friendly against Ireland on Tuesday.
Flick said Joachim Low's side were preparing for Ronaldo to definitely be part of the Portugal team for the meeting at the Arena Fonte Nova in Salvador.
"We think that Cristiano Ronaldo will be available on Monday - full stop," he told a news conference on Monday.
"He has not been voted World Player of the Year for no reason. He has a fantastic goalscoring record.
"He is the kind of player that can decide a match on his own so we firmly believe he will be available for the game against Germany.
"I think you can compare him to (Germany goalkeeper) Manuel Neuer's preparations with physiotherapy, leaving it right until the last minute.
"I don't think Portugal will be prepared to do without him even if there was a slight chance of him being fit. We think he'll be there."
Flick said Germany were 'cautiously positive' heading into their first game of the tournament, knowing a good start was crucial.
"Even without Cristiano Ronaldo, Portugal are a mighty opponent, definitely one we will take very seriously with great respect but we also know our own strengths," he said.
"We approach this game with self-confidence and we're looking forward to the first game because everybody knows the first match in a tournament can be the make or break how your tournament goes.
"We're cautiously positive ahead of the first match."
