Hydrolysis is an important degradation pathway in biodegradable polymers exposed to water. Hydrolytic chain scission leads to molecular weight reduction and mass loss, which in turn impact the mechanical properties. Conversely, mechanical loads applied during degradation can impact the degradation rate. Capturing chemo-mechanical couplings in biodegradable polymers is particularly important in biomedical applications (orthopaedic fixtures, tissue engineering scaffolds), where the mechanical properties directly impact the healing outcomes. In this work, we develop a thermodynamically consistent constitutive framework coupling large elasto-viscoplastic deformations, hydrolytic damage and mass transport in amorphous polymers subjected to mechanical loads in water. Hydrolytic chain scission is described building on our recent theory [1], which accounts for different chain scission mechanisms (random scission, chain-end scission, or any combination of both) and autocatalysis. The viscoplastic behaviour is described as a thermally-activated process with an effective temperature capturing the effect of degradation on the mechanical properties via the decrease of the glass transition temperature. The constitutive model is validated against experimental data for the degradation of amorphous PLA subjected to degradation under loads. We also present finite element simulations to illustrate the effect of heterogeneous degradation on the mechanical response of biodegradable polymer structures.