Hawks coach Mike Budenholzer said Millsap will miss Tuesday's game against the Chicago Bulls and Wednesday's game against the Boston Celtics.
Millsap suffered a right knee contusion in Friday's loss to the Detroit Pistons and missed a game the next night against the New York Knicks.
The Hawks (26-29) are the eighth seed in the Eastern Conference but have gone 2-8 in their past 10 games.
Millsap is averaging 17.6 points, 8.3 rebounds and three assists per game in his first season in Atlanta. He signed a two-year, $19 million deal in the offseason.