The fracture was discovered during a routine postseason physical, a league source told the San Antonio Express-News, but Ginobili wrote on Twitter on Wednesday that he'll know more in the next 20 days.
"The injury exists," Ginobili tweeted in Spanish. "But my participation in the World Cup will depend on how much I get better in the next 20 days. I'm being optimistic.
"As soon as I get better, I'll be able to say more, sooner than later."
Ginobili, who is consulting with a foot specialist for a second opinion, could miss up to two months. He is expected to be ready for training camp in late September.
The injury was earlier reported by Yahoo! Sports.
Ginobili, 36, averaged 14.4 points and 4.4 assists per game during the Spurs' five-game series win over the Miami Heat in the Finals. He often was limping at the end of games, although he still averaged 28.6 minutes per game in that series.
Report: Ginobili Played Finals Injured
ESPN Insider Dr. Mark Adickes breaks down the report that Spurs guard Manu Ginobili played in the NBA Finals with a slight stress fracture in his right leg.