Their return is vital for Croatia's chances of progressing further in the tournament at the expense of Portugal and improving on its performances in 1996 and 2008, when it reached the quarterfinals.
The winner of Saturday's match plays Poland or Switzerland in the quarterfinals.
Modric is particularly crucial for Croatia in midfield, helping the team defensively and creating most of Croatia's chances. Croatia collapsed in the game with the Czechs after he was substituted in the 62nd minute.
Mandzukic, the Juventus striker, has scored 24 goals in 67 international games. Croatia lost to Portugal in all their previous three encounters, including a 3-0 defeat in the group stage at Euro 1996, and has failed to score a single goal.
Croatia has been in formidable form in France and advanced to the round of 16 as winner of Group D. But Cacic knows it won't be enough for his players to focus on their game because Portugal will be featuring one of the world's most dangerous strikers after Ronaldo rediscovered his scoring touch with two goals against Hungary.
"We have to defend well against Portugal," Cacic said. "This is a team with great individual qualities. Of course, Ronaldo is their main player. So what we have to do tomorrow is not to allow them to show how strong he is. We have to limit Cristiano Ronaldo."
