The youngster impressed in the early part of the season after moving to Spain, but his form has dipped in recent months following off-the-field concerns.
However, Carlos believes that the 22-year-old will fire the Selecao to glory at the World Cup this summer.
He told Bild: "[I think we will win] not only because of the advantage of playing at home but because Luiz Felipe Scolari changed the way we play football in the Confederations Cup last summer.
"Neymar is a genius. He will be a key player at the World Cup. What is most striking about him is his character. Players with good judgement and confidence are needed."
Carlos made 125 appearances for his country between 1992 and 2006.