"Neymar had a pain in his left ankle after finishing training on Thursday and tests have shown that the player has a grade one sprain," a statement read.
The Brazilian was only given the all-clear by medical staff at the Camp Nou on Monday after fracturing a vertebra playing for his country in the World Cup quarter-final against Colombia in early July.
Neymar looked lively in Barca's final friendly match against Mexican outfit Leon on Monday in which they won 6-0 and he bagged a brace.
Barca kick off the season at home to Elche under new coach Luis Enrique having had an overhaul of the team following their first season without a major trophy in six years.
Luis Suarez leads the list of arrivals after a 65 million pounds ($107.76 million) transfer from Liverpool but he will not be available until late October due to his ban for biting at the World Cup.