A relevant issue in managing a complex software system is the quantitative assessment of the fulfillment degree of its requirements, as it provides a sound basis for identifying suitable design time or runtime corrective actions. However, this quantitative assessment must cope with the uncertainty that could affect the requirements specification and the data used in the assessment process. We tackle this problem, assuming the system requirements are specified through a hierarchical goal model. For this case, we propose an approach rooted in the fuzzy set and fuzzy logic theory for the quantitative assessment of the fulfillment degree of the goals specified in the goal model, considering the different kinds of uncertainty that could be present. Our result enriches the methodological and practical toolset available for managing complex software systems in uncertain scenarios, complementing other approaches based on different formal frameworks like probability theory.