Durant, who complained of pain after a Saturday workout, will require surgery for the break at the base of the small toe, which typically takes six to eight weeks of recovery time but could take longer depending on the severity of Durant's specific injury.
The 26-year-old forward figures to be out for at least the first six weeks of the upcoming NBA season, about 20 games or nearly a quarter of the season.
"After practice yesterday, Kevin made us aware of discomfort in his right foot," Thunder general manager Sam Presti said. "We proceeded to perform the necessary imaging studies to determine the cause of his discomfort.
"We are in the process of collaboratively evaluating the most appropriate next steps with Kevin, his representatives, and Thunder medical personnel. Until a course of action is determined, we are unable to provide a timeline specific to Kevin's case."
Presti said the team was lucky to have caught the injury quickly.
That could make the difference in getting Durant back by the time the Thunder face the NBA's top superstar, LeBron James, and the Cleveland Cavaliers at home on December 11 in what typically would be one the season's most eagerly anticipated matchups.
Durant averaged 32 points, 7.4 rebounds and 5.5 assists a game last season, sparking the Thunder to the Western Conference finals, where they were eliminated by the eventual NBA champion San Antonio Spurs.
Last year, the Thunder were hampered by an injury to guard Russell Westbrook much of the season. Durant picked up much of the slack in Westbrook's absence and the reverse will likely be needed for Oklahoma City early in the 2014-15 campaign.
"In these cases, we can withdraw or advance and we're going to advance," Presti said. "I have confidence in the team that we've built here. How we react, how we respond, how we adjust, how we adapt, that's going to be the measure of our team."
Durant, who guided Oklahoma City into the 2012 NBA Finals, where the Thunder were beaten by the Miami Heat, set an Olympic scoring record with 19.5 points a game in 2012 and added 5.8 rebounds, 2.6 assists and 1.6 steals a game to spark the Americans to gold at London.
Durant, who helped the US win the 2010 world title, pulled out of this year's world championships squad in August, citing mental and physical exhaustion. The Americans went on to defend their crown in Spain.
He pulled out less than a week after Indiana Pacers star Paul George suffered a broken leg in a team scrimmage at Las Vegas that will keep him out for the entire season.
"Get well KD! Speedy recovery," George tweeted Sunday.
Durant had scored 23 points in two pre-season games for the Thunder, playing only about 30 minutes in all.
After winning last season's scoring crown, he joined Michael Jordan, Wilt Chamberlain and George Gervin as the only players to win four NBA scoring titles in five years.
In 542 regular-season games over seven NBA seasons, Durant has averaged 27.4 points and 6.9 rebounds.
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