Fernando Santos' men return to action just three days after their draw with Hungary for a meeting with the Group D winners in Lens on Saturday.
While Croatia had an extra day of rest, Joao Mario said that would have no impact at the Stade Bollaert-Delelis.
"We know we haven't much time to recover," he told a news conference. "It's not ideal but we have to go onto the pitch and represent our country.
"In the knockout stage of a competition like this one, you always feel energised and I'm sure we'll be ready."
With two goals each, Cristiano Ronaldo and Nani have scored all of the team's goals at the competition so far, but Sporting midfielder Joao Mario has denied suggestions that Portugal lacked a cutting edge.
"These matches are always very tactical, all the teams defend very well," the 23-year-old said.
"But I don't think we've been remiss and proof is that we scored three goals in our last match. I don't think we've lacked creativity."
Portugal have won all three previous meetings with Croatia, who have failed to score a goal against the Seleccao, and Joao Mario is confident his nation can maintain that unblemished record.
"They'll have to run more than us and fight more than us to beat us and I don't think they'll be able to do that," he said.
"We believe that we will make it through."