Recycling of thermoplastic polymers is an important element of sustainable circular economy practices. The quality of mechanically recycled polymers is a concern. A method is presented to predict the structure and processability of recycled blends of polymers based on processability knowledge of their virgin precursor components. Blending rules at molecular weight distribution level are well established and form the foundation of the new method. Two essential fundamental building blocks are combined with this foundation. First, component and blend structure are related to viscosity via tube theories. Second, viscosity is related to melt flow index via a continuum mechanics approach. Emulator equations are built based on virtual experimental designs for fast forward and reverse calculations directly relating structure to viscosity and processability. The new combined method is compared with empirical blend rules, and shows important similarities and also clear quantitative differences. Finally, the new method is applied to practical recycling quality challenges.