The 7-footer, who had been slow to recover from a torn anterior cruciate ligament in his right knee that cost him all of last season, will undergo surgery March 4 to repair a torn lateral meniscus in the same knee.
Seattle's 12th overall pick in the 2004 draft hurt the knee in a game at Portland on Feb. 21.
The team said in a statement Friday that Swift received a second opinion on the latest injury this week from Dr. Steve Lombardo in Los Angeles.
The Sonics had said they wanted to spend the remainder of the season evaluating how Swift fits into their remodeling plans. Seattle recently traded forward-center Kurt Thomas to San Antonio partly to give Swift and Johan Petro, another inexperienced center, room to grow.
"Robert is an important part of the Sonics, and we will continue to support him throughout this rehabilitation process," general manager Sam Presti said last week.
Even before he knew for sure that Swift was headed for another surgery, Sonics coach P.J. Carlesimo called Swift's injury "horrible news."
Also Friday, the Sonics waived veteran forward Ira Newble. Seattle had acquired Newble on Feb. 21 from Cleveland in the three-team trade that sent Wally Szczerbiak to the Cavaliers. Newble played 17 minutes in two games for the Sonics.
Later Friday, the Sonics signed guard Mike Wilks to a 10-day contract.
Wilks played briefly earlier this season with Denver and Washington, but appeared in 47 games last year with the Sonics and averaged 3.6 points. He could see immediate playing time because starting point guard Earl Watson has missed the last two games with an illness.