The two sides have met 10 times with their last encounter ending in a goalless draw in 2012
Portugal's coach Fernando Santos will leave his final decision on team selection for the quarter-final clash against Poland until today morning because of injuries to at least two players, he told reporters.
Speaking on the eve of the game in Marseille, Santos also said he viewed the two sides as evenly matched which could result in a tight, defensive contest.
"There are some players who aren't 100 per cent fit so we'll wait until tomorrow to ascertain whether they are entirely fit or not and if they can play or not," Santos said.
"Sometimes the players can recover and we still have 24 hours to go."
Defender Raphael Guerreiro and midfielder Andre Gomes have not trained since Monday and on Wednesday were again not on the pitch with the rest of the squad.
The two sides have met 10 times with their last encounter ending in a goalless draw in 2012 and the omens are that it could be another close game.
Apart from a stunning bicycle kick goal by Switzerland's Xherdan Shaqiri, the Polish defence has not been breached.
Portugal's last five European Championship ties have ended in draws after 90 minutes, although they scraped through against Croatia in the last 16 with a 117th minute winner.
"I think it's a 50-50 match. My players know that. They (Poland) are here on their own merit," Santos said.
Asked if it could be similar to the Croatia clash, Santos lauded Poland's defensive unity and collective spirit.
"Of course, it will be a game of a lot of strategy. Sometimes there is a Gordion Knot that you can't untie," he said, referring to the mythological legend used as a metaphor for a difficult problem.
"There will be a strong strategic component to this game, because if you concede four goals nobody can ensure you can score five," he quipped.
Talisman Cristiano Ronaldo struggled against Croatia producing one shot, but Santos defended his "amazing" captain.
"We want to strike a balance, of course. We want Ronaldo to shoot on target 15 times and score 10 goals, but our opponents won't let us and that's part of the game," Santos said.
There have also been concerns over the Stade Velodrome pitch, but Santos, whose country lost to France in the 1984 European Championship semi-final in Marseille, said it looked better than expected.
"We've thought about the match here in 1984, but we played against France so you can't compare the two. It's apples and oranges," he said.