Indiana Pacers All Star forward Paul George suffered an ugly leg injury while playing in a Team USA practice game at the University of Nevada in Las Vegas on Friday.
The 24-year-old landed awkwardly on his right leg after contesting a layup attempt by James Harden of the Houston Rockets early in the fourth quarter.
Several players, including Chicago Bulls point guard Derrick Rose, were visibly upset as George received medical treatment before being taken off the court on a stretcher.
USA Basketball head coach Mike Krzyzewski, who has steered the U.S. team to the past two Olympic gold medals, then told the crowd at the Thomas & Mack Center that the intra-squad game would not continue.
"It's a tough situation for our entire organization. Our coaches and players are very, very emotional," USA Basketball Chairman Jerry Colangelo told reporters.
"There's no way the game could have gone on under the circumstances."
NBA Commissioner Adam Silver later said in a statement: "It was difficult to watch the injury that Paul George sustained tonight while representing his country. The thoughts and prayers of all of us at the NBA are with Paul and his family."
LeBron James, the NBA's biggest star, tweeted: "Man just landed and got a message about @Paul_George24. Made me immediately sick to my stomach!! Praying for u a speedy recovery homie!!!"
Larry Bird, the Indiana Pacers president of basketball operations, said in a statement: "We are aware of the injury sustained by Paul George in Friday night's Team USA game in Las Vegas and we are obviously greatly concerned.
"At this time our thoughts and prayers are with Paul. We are still gathering details and will have an updated statement Saturday."
USA Basketball will be selecting a team to defend their 2010 world title at the World Cup in Spain beginning on Aug. 30.
They are aiming to become only the third team after Brazil (1959 and 1963) and Yugoslavia (1998 and 2002) to win back-to-back world titles.