Williamson averaged a heady 23.3 points and 6.5 rebounds in just 27.3 minutes per game — while shooting a robust 71.4 percent from the field — in his four preseason appearances. But Pelicans Coach Alvin Gentry told reporters on Friday morning after the team’s shootaround at Madison Square Garden that Williamson is believed to have suffered the injury on Oct. 13 against the San Antonio Spurs. Gentry would not specify how long the team expects Williamson to be out.

The Pelicans left Williamson behind in New Orleans on Thursday for further evaluation and treatment when they traveled to New York for their preseason finale on Friday night against the Knicks. ESPN reported on Friday that Williamson “has avoided a serious injury to his right knee but is expected to miss a period of weeks.”

This will be the second injury absence of Williamson’s nascent pro career. He lasted just nine minutes in his summer league debut in July in Las Vegas after banging knees with an opposing player, resulting in an injured left knee that prompted the Pelicans to hold him out of the rest of the summer schedule to be cautious.

Williamson bounced back in exhibition play with a level of production and efficiency that quickly established him as an overwhelming favorite among oddsmakers and N.B.A. pundits to win rookie of the year honors.