Subitizing, a quick apprehension of the numerosity of a small set of items, has been found to explain students' number understanding when counting. We utilized a constructivist teaching experiment methodology to investigate how the counting and subitizing activity of one student, Diego, related to his number understanding (described by his pre-numerical units construction) when solving early number tasks. Subitizing and counting tasks were designed to assess and leverage Diego's pre-numerical units development. Findings indicated that as Diego's pre-numerical units included more than one way to construct five, he still relied on external material. Implications of this study suggest that task design should be grounded in unique aspects of student reasoning and should be designed to promote students' partitioning as well as use of finger patterns, pointing, and number words.