An MRI confirmed the diagnosis, the team said. Lillard will be evaluated again in 10 days and will miss at least four upcoming games.
Lillard sat out Tuesday's 110-92 win over the Detroit Pistons to manage the injury. He has been dealing with abdominal pain since playing for Team USA at the Olympics this summer.
Lillard has struggled to start the season but has maintained that injury is not the reason for his recent stretch of poor play. Through 20 games, Lillard is averaging 21.5 points while shooting 39.7% from the field, including just 30.2% from 3-point range. He also is averaging 4.0 rebounds, 7.8 assists and 35.7 minutes.
Portland forward Nassir Little is also out for at least one week with a left ankle sprain. Little was injured Monday against Utah.
Little has played in 21 games, including two starts this season, and has averaged 8.3 points and 5.3 rebounds.
The Associated Press contributed to this report.