Modular product design has become a strategic enabler for companies seeking to balance product variety, operational efficiency, and market responsiveness, making alignment between modular architecture and manufacturing considerations increasingly critical. Modular Function Deployment (MFD) is a widely adopted method for defining modular product architectures, yet it lacks systematic support for assembly considerations during early concept and system-level development. This limitation increases the risk of delayed production ramp-up and life cycle inefficiencies. This paper proposes a set of enhancements to MFD that integrate Design for Assembly (DfA) logic into architectural synthesis. The extended method introduces structured heuristics, assembly-oriented module drivers, a coded interface taxonomy, and quantitative metrics for assessing assembly feasibility and automation readiness. These additions preserve compatibility with standard MFD workflows while enriching decision-making with traceable, production-informed reasoning. An illustrative case study involving a handheld leaf blower demonstrates the method's usability and effectiveness. The redesigned architecture shows reduced assembly effort, simplified interfaces, and increased automation potential. By supporting early-stage evaluation of architectural alternatives through an assembly lens, the method enables faster transition to efficient volume production and provides a foundation for continuous improvement throughout the product life cycle.