Canada's environment ministry has issued a warning that temperatures could reach 35* Celsius (95* Fahrenheit) on Thursday in the city, but it could feel like 44* Celsius (111* Farenheit) due to elevated humidity.
"I don't like to speak about this, or use it as an excuse," Portuguese midfielder Vitinha told reporters, when asked about the impact of extreme heat on play.
Environment Canada said temperatures may ease by kickoff at 7:00 pm (23:00 GMT), although the risk of thunderstorms could impact the match.
Portugal made an unconvincing start to their World Cup campaign but Paris Saint-Germain midfielder Vitinha said he received the criticism of their performance "with open arms".
Portugal has fallen into a tougher draw after failing to beat the Democratic Republic of Congo and Colombia, with a rout of low-ranked Uzbekistan their lone victory in the tournament's group stage.
"We know that there is hardly any room for mistakes, if any," he said. "I hope that the collective group will perform better."
The contest between the two top European sides will almost certainly mark the final World Cup appearance for either Portugal's Cristiano Ronaldo or Croatia's Luka Modric, who for so long were teammates at Real Madrid.
Vitinha said his side has "a lot of motives... to want to win tomorrow," and extending Ronaldo's World Cup stay was top among them.
He also hailed Modric as "a great reference for all football players and even more to me."
"It's a pleasure to play against him and hopefully tomorrow he'll be a little bit sadder than me," Vitinha said.
"We have to stop a team tomorrow, and hopefully that will be the team of Luka Modric."
Croatia coach Zlatko Dalic said he expected Thursday's match to be "a midfield battle."
"We need to reduce the errors to a minimum because they will punish us just as England did and we need to raise the level of responsibility in the midfield," Dalic said, referring to the 4-2 defeat in their opening game.