The Brazil star - who has only just returned from a back injury sustained at the World Cup – scored twice in Barcelona's 6-0 friendly win over Leon on Monday.
But the 22-year-old could miss the beginning of Barca's league campaign after the Spanish club confirmed on Friday that he was battling an ankle issue.
"Following the Thursday training session, Neymar Jr. complained about some pain his left ankle," a club statement read.
"And tests have confirmed the player has a grade one ankle sprain. The club's medical team has indicated that his return to training will depend on how the injury treatment evolves."
Neymar was withdrawn from Friday's scheduled press conference as a result.
Neymar failed to live up to the expectation after his big-money move from Santos last term, scoring just nine times in 26 Liga appearances.
But new Barca coach Luis Enrique will be desperate for his stint on the sidelines to be short, particularly with fellow forward Luis Suarez banned until late October for biting at the World Cup.