We theoretically analyze the effect of inter-subband couplings on the eigenstates of a terahertz heterostructure in the presence of coulombic donors. We find that the inter-subband electron-donor interaction causes a drastic reorganization of the eigenenergies and a strong admixture between the subbands that decisively influences the optical absorption spectrum of the structure. These results illustrate the importance of the inter-subband couplings in the formulation of the eigenstates of imperfect terahertz heterostructures.