Daniel Gafford will miss at least the next six weeks with a Grade 3 MCL sprain, sources told Shams Charania and Tim MacMahon of ESPN.
The Mavs are already without Anthony Davis, Dereck Lively II, and Dwight Powell.
Davis is expected to miss at least four weeks with an adductor strain.
Lively remains in a walking boot due to a stress fracture in his ankle.