Williams, the Hawks' third leading scorer with a 14.1 points average, underwent a scan and examination in Atlanta which revealed the injury to his right knee.
The eighth-year player will need surgery and only then will a full timetable for his recovery be known, the team said.
"We are extremely disappointed for Lou," said Hawks general manager Danny Ferry.
"He has been a complete professional in every sense of the word and know that approach, along with his work ethic, will serve him well in his rehabilitation."
(Reporting by Simon Evans in Miami; editing by Gene Cherry)
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