Neymar, 34, had only recently returned to the national team setup after a long spell out following a serious ACL injury suffered in a friendly against Uruguay in October 2023.
His inclusion in the World Cup squad was seen as a major boost for Brazil, despite concerns over his fitness.
However, those concerns have now been realised. Neymar picked up a calf injury just weeks before the tournament, which has disrupted his preparations and delayed his return to full training.
Speaking ahead of Brazil's opening Group C fixture, Ancelotti was clear about the situation but remained hopeful over the player's recovery.
"He is working hard to be fit again as quickly as possible. We hope he can return to full training next week," Ancelotti said.
Brazil face a challenging start to their World Cup campaign against Morocco, a fixture widely expected to be their toughest in the group stage. They will also meet Haiti and Scotland in Group C as they aim to progress to the knockout rounds.
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