Razorbacks coach Mike Anderson said Tuesday that an MRI revealed torn ligaments and that Powell will require surgery. His timetable for a return is six to eight months.
The school did not disclose which knee was injured Nov. 17 during practice. Powell appeared to favor his right knee the next day as he watched the Razorbacks lose to Houston.
"This is a big blow to Marshawn and our basketball team," Anderson said. "He has been working hard on and off the court this year and was in the best shape of his career."
The 6-foot-7 Powell was a preseason second-team All-Southeastern Conference pick and averaged 19.5 points and six rebounds through the first two games, shooting 71.4 percent.
The junior averaged 14.9 points as a freshman before a foot injury limited him to 10.8 points last season.
Powell talked openly in the preseason and admitted that his own poor attitude following the injury last season hampered his play. He was thankful for the hiring of Anderson and the chance at a new beginning with the Razorbacks, shedding nearly 30 pounds while getting in better shape under Anderson's guidance.
Powell opened the season with a 19-point effort against South Carolina-Upstate, and he followed that up by scoring 18 of his game-high 20 points in the first half of a win over Oakland, Mich.
"This is definitely a setback, but I told him that sometimes you have to have setbacks in order to have a great comeback," Anderson said. "He is in good spirits and looking forward to helping his team in any way that he can."
Arkansas had just 10 scholarship players this season before Powell's injury, and Anderson now must find a way to replace his top player and the depth needed to run his up-tempo style of play. The injury was announced hours before the Razorbacks hosted Utah Valley.