Dutt said they aren't certain yet about the severity of the injury or how long Lewis will be sidelined.
The Mavs signed Lewis, 34, a 16-year veteran, to back up Dirk Nowitzki at power forward. The two-time All-Star averaged 4.5 points and 1.8 rebounds in 16.2 minutes per game for the Miami Heat last season, primarily coming off the bench.
Yahoo! Sports earlier reported that Lewis needed surgery.