Degeneration of intervertebral discs (IVDs) is accompanied by concurrent changes in geometry and intrinsic tissue properties, which are often treated separately in continuum-mechanical modeling approaches for reasons of tractability. However, whether degeneration-related mechanics can be reduced to either geometric or constitutive adaptation alone remains insufficiently understood. In this study, we investigated the mechanical consequences of such reductions using finite element modeling (FEM) of lumbar IVDs. A hexahedral FE model with a Holzapfel-Gasser-Ogden formulation was examined in two different settings: A) geometry-only degeneration using MRI-derived, patient-specific lumbar discs (ndeg=13) with material properties of a disc-healthy population [1], and B) material-only degeneration via constitutive calibration on a healthy-height reference configuration (“source mesh”) [2]. Simulations under flexion, extension, lateral bending, and axial rotation were compared against in vitro range of motion (ROM) data of healthy and severely degenerated discs. For geometry-only degeneration (A), numerical models systematically underpredicted the absolute ROM across all loading modes, with the largest discrepancies observed in lateral bending (RMSE≈1.6°). While height loss reproduced qualitative degeneration trends, it failed to capture the magnitude of degeneration-related ROM reductions. For material-only degeneration (B), 30 independent calibration attempts of the healthy-height source mesh were evaluated. While flexion and extension could be matched in isolation, no parameter configuration reproduced ROM consistently across all loading modes, with the best-performing solution still yielding a worst-mode R^2≈-1. This indicates systematic misrepresentation of at least one loading direction. Together, these findings provide continuum-mechanical evidence that disc degeneration constitutes a coupled geometric–constitutive process and cannot be reduced to either component alone. They furthermore motivate degeneration-dependent material calibration on geometrically consistent, height-reduced source mesh configurations as a necessary step toward mechanically realistic, personalized intervertebral disc models.