The Brazilian Football Confederation (CBF) confirmed the 34-year-old forward, who is recovering from an injury in his right calf, did not take part in the closed session at their facilities in Granja Comary and was referred to a private clinic in Teresopolis for imaging tests.
In a statement, the CBF said "no further information will be released until the Brazilian national team's medical staff have completed their assessments," but Neymar's absence quickly became the dominant storyline on the second day of Brazil's training camp for the 2026 World Cup.
Brazil will hold three further sessions at Granja Comary before Sunday's friendly against Panama at the Maracana.
Manager Carlo Ancelotti is already without three players for that fixture. Defenders Gabriel Magalhaes and Marquinhos, along with forward Gabriel Martinelli, are involved in Saturday's Champions League final between Arsenal and Paris Saint-Germain.
Neymar's recall last week generated widespread excitement because he had not featured in Ancelotti's plans during the Italian's year in charge.
Brazil's all-time leading scorer with 79 goals in 128 appearances, Neymar has not played for his country since 2023. His return to the fold came amid scrutiny over his fitness and form following years of injury trouble and an underwhelming spell back at Santos.
The timing could hardly be more delicate. After facing Panama, Brazil meet Egypt in Cleveland in their final friendly before opening their World Cup campaign against African champions Morocco on June 13 in New Jersey. Brazil and Morocco have been drawn in Group C alongside Haiti and Scotland.
In an interview with Reuters in early May, Ancelotti made clear that reputation alone would not secure Neymar's place.
He said that Neymar would receive no special treatment and that his place in the squad would be strictly based on fitness and form, not sentiment.
For now, Brazil wait on the medical verdict - and on whether their most recognisable name will be fit to take centre stage next month.