Millsap, a first-time All-Star selection averaging 17.6 points and 8.3 rebounds in his first season with Atlanta, was injured late in the fourth quarter Friday at Detroit and missed the team's win against the New York Knicks the following day.
An MRI was negative.
The Hawks also say center Pero Antic, who has missed the past 14 games due to a stress fracture in his right ankle, also will be held out against the Bulls and Celtics. Center Gustavo Ayon will miss his fourth straight game against the Bulls with a right shoulder injury.
Antic and Ayon were the replacements for Al Horford, who is recovering from season-ending surgery for his torn right pectoral muscle.
The Hawks ended an eight-game losing streak by beating the Knicks on Saturday night.
Information from the Associated Press was used in this report.