The two nations had been considered the strongest teams in Group F, but the pressure on both has intensified after disappointing opening games.
Portugal could only draw 1-1 with tournament debutants Iceland, while Austria lost 2-0 to unfancied Hungary.
The prospect of stopping the three-times world player of the year has added to Austria’s worries.
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"It will be a tough challenge. I had a few games against him with Schalke,” captain and left back Christian Fuchs told a news conference.
"He's the type of player you can never defend against for the whole game. You have to defend as a team. You need to avoid one-on-one situations (and) need to always get two versus one or even more," Fuchs told reporters ahead of the game at the Parc des Princes in Paris.
"But then you might forget about the other players. This team is not only Ronaldo. Take Ronaldo out the game, the next player can decide the game too"
Austria's opening match defeat extends a run of poor form, which includes friendly losses to Albania, Turkey and the Netherlands.
That compares with a near-perfect qualification record of nine wins and a draw in which forward Marc Janko netted seven times.
Fuchs, whose team have only ever scored one goal at a European championship, bristled when asked whether the slump would continue against Portugal.
"If we had (that) attitude we couldn't have any success at all," he said. "Now we will fight harder than ever. I think tomorrow we will be a different team and we will have a lot of bite.”
Defender Aleksandar Dragovic will be suspended following his dismissal against Hungary. Midfielder Zlatko Junuzovic suffered an ankle injury and will also be absent.
Coach Marcel Koller refused to be drawn on who would replace them or on any other potential changes.
Fuchs said Austria would stick to their attacking principles despite the danger Ronaldo and unpredictable forward Nani could pose.
"It's not our way of playing to defend only. We like to play a good game, we like to play football,” added Fuchs. "We want to win this game."
(Editing by John Irish)
