Du Preez is expected to at least miss this weekend's all-South African clash with the Cheetahs, with his club doctor suggesting he could face a lengthier lay off.
Bulls doctor Org Strauss has hinted at the possibility of a torn calf muscle, which could lead to up to two months on the sidelines.
Although Du Preez's injury is not expected to be too serious, any absence will come as a blow to Bok head coach Peter de Villiers as he looks to keep his players fit and in form ahead of the three-match series with the Lions.
