Pelé's first game in the World Cup was against the Soviet Union. It was a 2-0 victory, with two goals from Vavá, who had already made his debut in the previous game against England. Diego Maradona made his World Cup debut for Argentina without scoring, in a 1-0 defeat to Belgium. Lionel Messi scored once against Serbia and Montenegro in his first game. Cristiano Ronaldo drew a blank against Angola in his debut, as did Michel Platini, Zico, Garrincha, Johan Cruyff, Roberto Baggio ...
The last players to debut with two goals in the World Cup were Tim Cahill, in Australia's 3-1 win over Japan, and Tomas Rosicky, in the Czech Republic's 3-0 win against the United States, in Germany 2006. Mario Mandzukic matched them. All the great players mentioned above were much better than the Croatian striker, obviously. But none of them achieved what he managed to in their first World Cup games: two goals.
Ferenc Puskas did it. Ronaldo "Fenômeno" didn't. Nor did Romário, Careca, Reinaldo.
It's rarer than you think. Mandzukic would have made Brazil's game against Croatia more difficult. His absence against the Seleção helped, as did his presence against Cameroon. His two goals helped to eliminate Cameroon, who would have been more motivated for Brasília, for a game that Brazil must win to likely finish first in the group and advance.
Cameroon deploy a 4-3-3 formation and offer more space to play than either Croatia or Mexico, two respectable opponents -- don't forget that Australia made a strong showing against the Netherlands. There may be more space for Hulk to cut in from the right flank, Neymar from the left, or for Paulinho to ghost in as he likes to. Who knows, it could be a game to give Brazil the confidence they need before progressing to the round of 16.