The Chelsea starlet scored Brazil's third goal in a 3-1 win over Croatia in Thursday's World Cup opening match, and also teed up Neymar to crash in the equaliser after the tournament hosts had fallen behind to an early Marcelo own goal.
Local media had suggested Oscar's Stamford Bridge team-mate Willian would start Thursday's curtain raiser instead, but Cafu - who played in four World Cups, winning the competition twice - insists the right decision was made.
"Oscar is a great player, which is why [Scolari] took the risk in starting him over Willian," Cafu told journalists at the adidas Dugout in Rio de Janeiro.
"He was the best player on the pitch - not just because he scored a goal, but made an important pass and played with a lot of personality. He knew exactly what the manager wanted him to do.
"After his fantastic performance he's now an irreplaceable player in the team. It will be hard for the other players - like Willian - to take his place."
Cafu also spoke of the controversial penalty which helped Brazil turn the tide in the tournament's opening match.
With the score at 1-1, Fluminense striker Fred appeared to go to ground rather easily under a challenge from Dejan Lovren, but Cafu insists Brazil were playing well enough to win the match regardless of that seemingly generous award.
"That decision wasn't why Brazil won the game," Cafu said. "Sometimes referees do give these decisions and sometimes they don't. What's more important is the performance, and Brazil played very well."
Brazil's second match in Group A is against Mexico on Tuesday.
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