During a workout Thursday, Roberson came down from a jump and reported some discomfort in his knee. An MRI revealed a small avulsion fracture. He'll be re-evaluated in six weeks.
An avulsion fracture occurs when a tendon or ligament pulls off a piece of bone.
Roberson ruptured his left patellar tendon in January and initially was on track to return in November, but he had a setback in early October when a suture caused irritation and required a procedure.
Roberson has been on the court in recent days, going through increasingly intense post-practice workouts -- cutting, jumping, dunking and running. He was still some time away from being available to play.
Despite not having Roberson, known as one of the league's top perimeter defenders, the Thunder have the NBA's top defense, allowing just 101.8 points per 100 possessions.
Without Roberson, the Thunder have leaned on a collection of wings including Terrance Ferguson, Alex Abrines and rookie Hamidou Diallo. Both Ferguson and Diallo have missed the past three games because of ankle sprains but are expected to return in the next few days.