During workflow design, workflow modelers generally specify the performers of a task by their organizational role. However, during workflow enactment, numerous workers with different skills and expertise may share the same role in an organization, making it hard to select appropriate individuals based merely on the assignment relation between a role and a task. To bridge the gap between abstract roles and real workers, this work proposes a multi-criteria assessment model capable of evaluating the suitability of individual workers for a specified task according to their capabilities, social relationships, and existing tasks. Candidates are ranked based on their suitability scores to support workflow administrators in selecting appropriate workers to perform the tasks assigned to a given role. The proposed assessment model overcomes the lack of role-based task assignment in current workflow management systems.