The adoption of a lean production model means a systematic implementation of various management methods and practices. Such a model presents the human element as a key factor in continuous improvement efforts, influencing workers’ job content and the quality of work. However, there are few evidences regarding research on quantitative assessment of the work demand, either psychological or physical, in a lean production environment. Therefore, this study aims to analyze from an ergonomics perspective the psychophysical demand and workers’ perceived workload within assembly cells undergoing a lean production implementation. The proposed method integrates complementary concepts of widely known techniques, enabling the consolidation of several assessment criteria into one particular index for both psychological and physical demands. Such a diagnostic method is illustrated in a case study from the automotive parts manufacturing sector, whose application is performed in three critical assembly cells selected by senior management. Our findings show systemic gaps between work demand and employees’ profile, indicating improvement opportunities in order to provide a better work environment.