The Architecture & Supplier Identification Tool (ASIT) is a design support tool, which enables identification of the most suitable architectures and suppliers in early stages of complex systems design, with consideration of overall requirements satisfaction and uncertainty. During uncertainty estimation, several types of uncertainties that are essential in early design (i.e., uncertainty of modules due to new technology integration, compatibility between modules, and supplier performance uncertainty) have been considered in ASIT. However, it remains unclear whether expert estimation uncertainties should be taken into account. From one perspective, expert estimation uncertainties may significantly influence the overall uncertainty since early complex systems design greatly depends on expert estimation; while an opposing perspective argues that expert estimation uncertainties should be neglected because they are relatively much smaller in scale. In order to understand how expert estimation uncertainties (especially subjective uncertainty) influence the architecture and supplier identification results achieved through ASIT, a comprehensive study of possible modelling approaches has been discussed; and type-1 fuzzy sets and 2-tuple fuzzy linguistic representation are selected to integrate subjective uncertainties into ASIT. A powertrain design case is used to compare results between considering subjective uncertainties and not considering them. Finally, the consideration of subjective uncertainty in early conceptual design as well as other design stages is discussed.