"They're going to operate on me," the 31-year-old told Argentina's La Nacion newspaper Friday.
Ginobili, who helped lead Argentina to a bronze medal in the Beijing Olympics, said an MRI exam of the injury showed no improvement from a previous one.
"It's the same as it was two months ago, when they did the first exam," he said. "It's not worse, which is important. Now, the thing is, it's not better either, and it seems like the only way to repair it completely is arthroscopic surgery."
Ginobili will have surgery next week in Los Angeles, according to a person close to Ginobili who wanted to remain anonymous.
In a statement Friday, the Spurs confirmed that Ginobili would undergo a procedure "to correct a posterior impingement of his left ankle."
"Following the conclusion of the Olympic Games Ginobili returned to San Antonio," the team's statement read. "An MRI conducted earlier this week confirmed that he had re-aggravated the injury and that there was no additional damage to the ankle or the ligament."
The Spurs had no additional comment, other than to say they will announce a timeline for Ginobili's return after he has surgery.
The injury hobbled Ginobili during the NBA playoffs, particularly during the Western Conference finals against the Los Angeles Lakers, when his signature explosiveness was visibly absent. Ginobili led the Spurs in scoring last season and won the league's sixth man award.
At the end of the season in late May, Ginobili had an injection in the ankle and said he expected it to improve quickly. Ginobili wore a walking boot for several weeks starting in June after an MRI exam showed a ligament to be five times the size of the one in his other foot.
Spurs coach Gregg Popovich had urged Ginobili not to compete in the Olympics unless the injury improved. He not only competed in the Games, but also carried the flag for Argentina in the opening ceremonies in Beijing.
During Argentina's semifinal game against the eventual gold-medal winner United States, Ginobili hobbled off the court during the first quarter after apparently aggravating the injury. He did not play in Argentina's bronze-medal win over Lithuania, which he watched in street clothes from the bench.
"Aside from this situation, I'm comfortable with the situation," he said. "My plan was to be part of the Olympic Games, and I knew that if I suffered from pain they would have to operate. This isn't something that took me by surprise."
Ginobili, who had been leading the Olympic basketball tournament in scoring with 20.3 points per game, went down on a play away from the ball, limped to the sideline and took off his left sneaker. He went to the locker room and did not return.
After the bronze-medal game, he told the San Antonio Express-News that he did not regret playing in the Olympics.
"It's worth it," he said, according to the newspaper. "I had the opportunity to come here and to play on a good level. I had bad luck in the last game, but I'm so happy and so proud for what these guys have done.
"It's hard to explain it. I'm part of this, and I'm very proud."
Ginobili averaged 17.8 points, 3.8 rebounds and 3.9 assists a game for the Spurs last season.
Information from The Associated Press was used in this report.