Butler was injured when he deflected a pass in the first quarter of Sunday night's 101-96 victory over Charlotte.
The Bulls announced on Tuesday that an MRI and subsequent examinations had confirmed the diagnosis of sprained thumb ligaments. Butler will miss practice this week and will be re-evaluated after Chicago plays Minnesota in St. Louis on Friday night.
Butler averaged a career-high 13.1 points and 4.9 rebounds in 67 games last season. He also is regarded as a strong perimeter defender.
Chicago opens the regular season on Oct. 29 at the New York Knicks.