After reports the 34-year-old forward could be out for three weeks, Ancelotti instead insisted Neymar was working well towards fitness for the June 13 match against Morocco in New Jersey.
"We believe he'll be back very soon and if he doesn't make it for the opener, he'll be there for the second game," said Italian Ancelotti.
Former Barcelona and PSG star Neymar, now at Brazilian club Santos, will miss Brazil's two pre-World Cup friendlies against Panama and Egypt.
Brazil play Panama Sunday at the Maracana stadium before setting up camp in New Jersey on Monday and playing a final warm-up against Egypt on June 6 in Cleveland.
Neymar's inclusion in Ancelotti's 26-man squad for the World Cup was something of a surprise as he had not been picked for the national team in almost three years due to a mixture of injuries and poor form.
However, Neymar is Brazil's all-time leading goalscorer with 79 goals in 128 appearances for the Selecao. This will be his fourth World Cup.