Fati suffered a contusion to his right hip as he took part in a session on Friday.
"He is out and his recovery will determine his availability," confirmed Barca in a statement.
The problem is expected to rule Fati out of Saturday's friendly against Gimnastic.
Barca are due to begin their league campaign on the weekend of September 26/27 at home to Villarreal, with Fati likely to receive a bigger role under new head coach Ronald Koeman this season.
The 17-year-old scored seven times in 24 league outings in 2019-20, just 11 of which were starts.
Fati made a magnificent start to his Spain career during the international break, becoming the country's youngest goalscorer in a Nations League match against Ukraine three days after making his debut against Germany.