An MRI on Sunday at New England Baptist Hospital led doctors to the diagnosis, easing fears of a worse injury after Irving was hobbled with just over four minutes remaining in the second quarter of the Los Angeles Clippers' 123-112 victory at Boston.
Irving scored 14 points in 14 minutes on the court before leaving with the injury, appearing to suffer the setback after multiple moves on screens away from the ball.
Irving is averaging 23.8 points, 7.0 assists and 4.9 rebounds for the Celtics, who are battling Philadelphia for fourth place in the Eastern Conference.
The 26-year-old Australian-born American, the first overall pick in the 2011 NBA Draft, underwent left knee surgery last April.
Irving, who helped Cleveland win the 2016 NBA title before being traded to Boston in 2017, helped the US group of NBA stars to 2016 Rio Olympic gold.