Development of injection devices for subcutaneous drug administration requires a detailed understanding of user capability and forces occurring during the drug administration process. Injection forces of concentrated protein therapeutics are influenced by syringe properties (e.g., needle diameter) and injection speed, and are driven by solution properties such as rheology. In the present study, it is demonstrated that concentrated protein therapeutics may show significantly reduced injection forces because of shear-thinning (non-Newtonian) behavior. A mathematical model was thus established to predict/correlate injection forces of Newtonian and non-Newtonian solutions with viscosity data from plate/cone rheometry. The model was verified experimentally by glide-force measurements of reference and surrogate solutions. Application of the suggested model was demonstrated for injection force measurements of concentrated protein solutions to determine viscosity data at high shear rates (3 × 104–1.6 × 105 s−1). By combining these data with viscosity data obtained by different viscosity methods (plate/cone and capillary rheometry), a viscosity–shear rate profile of the protein solution between 102 and 1.6 × 105 s−1 was obtained, which was mathematically described by the Carreau model. Characterization of rheological properties allows to accurately predict injection forces for different syringe–needle combinations as well as injection rates, thus supporting the development of injection devices for combination products.