The evolution of the defect microstructure in materials at high temperature is dominated by diffusion-mediated interactions between dislocations, cavities, and surfaces. This gives rise to complex nonlinear couplings between interstitial and vacancy-type dislocation loops, cavities, and the field of diffusing vacancies that adiabatically follows the evolution of microstructure. In our previous work, we developed a nonlocal model for the climb of curved dislocations and the morphological evolution of cavities during postirradiation annealing of structural components in nuclear reactors. We now expand the formalism to include the treatment of population of very small defects and dislocation loops that are below the experimental detection limit. These are taken into account through a mean field approach coupled with an explicit real-space treatment of larger-scale discrete defect clusters. We find that randomly distributed small defects screen diffusive interactions between larger discrete clusters, renormalizing the free diffusion Green's functions and transforming them into Yukawa-type propagators. The evolution of the coupled system is modelled self-consistently, showing how the defect microstructure evolves through a nonmonotonic variation of the distribution of sizes of dislocation loops and cavities, treated as discrete real-space objects.