The team announced on Monday afternoon that Williamson had arthroscopic surgery on his right knee to treat a torn meniscus. The Pelicans expect Williamson will miss six to eight weeks.
Williamson would miss approximately 20 games on the six-week timetable and another seven or so at eight weeks. He is slated to miss Anthony Davis' homecoming to New Orleans on Nov. 27 but should be cleared by the time the Pelicans play on Christmas Day at Denver.
New Orleans first announced the injury last Thursday, as Williamson didn't travel to New York City for the Pelicans' final preseason game against the Knicks.
Pelicans coach Alvin Gentry said last Friday the thought was Williamson injured the knee on Oct. 13 against the San Antonio Spurs. Williamson finished that game with 22 points and 10 rebounds, his only double-double of the preseason.
In four preseason games, Williamson averaged 23.3 points and 6.5 rebounds while shooting 71.4% from the floor.
Williamson, who shouted "Let's Dance" at the end of his draft interview moments after becoming the No. 1 pick, was part of a summer that brought new life into the Pelicans franchise as a whole.
His summer league debut was sold out in Las Vegas even though he only played eight minutes because of a bruise to his left knee. The 6-foot-7, 284-pound Williamson did suffer a mild right knee sprain during his only season at Duke. That injury occurred when his sneaker gave out just seconds into a game against North Carolina. He missed three weeks with the injury before returning in the ACC Tournament.
When the Pelicans open up the season on Tuesday night in Toronto, forward Brandon Ingram will slide into the starting spot vacated by Williamson, with veteran guard JJ Redick joining Lonzo Ball and Jrue Holiday in the backcourt. Derrick Favors, who missed the final preseason game, is slated to start at center.
Despite the loss of Williamson, Redick told reporters on Sunday afternoon it hasn't dampened the expectations of the season.
"It's still the start of the season. We still get to play real games. We get to compete," Redick said. "We have a deep team. We'll hold it down as best we can without him. Obviously we need him back and hopefully he takes his time and recovers fully."