Sexton, 22, hurt his knee Nov. 7 in a game against the New York Knicks. The Cavs didn't provide an update until Friday, when they said Sexton underwent surgery Wednesday in Atlanta.
He was second on the team in scoring at 16.0 points and averaged 3.3 rebounds and 2.1 assists in Cleveland's first 11 games.
His loss is a significant blow for the Cavs, who got off to a strong start but have been ravaged recently by injuries. Rookie forward Evan Mobley is expected to miss a month with an elbow sprain.
The Cavs were missing six regulars Thursday night in a loss to the Golden State Warriors.
The Associated Press contributed to this report.