The New York Times reported that Jeffries had surgery on his broken left wrist on Tuesday and will miss more time than originally expected.
"Any time you have a player out, I always think the worst anyway," Coach Isiah Thomas said. "Our team is prepared to play for a long stretch without him."
The forward broke the wrist two weeks ago, and the team said at that time that he would miss six to eight weeks. The new timetable calls for him to miss six to eight weeks from the time of Tuesday's surgery.
Jeffries was signed away from the Washington Wizards this offseason for five years and $30 million.