Colloidal nanocrystals (NCs) are active materials in different applications, wherein their shape dictates their properties, such as optical or catalytic properties, and, thus, their performance. Hence, learning to tune the NC shape is an important goal in chemistry, with implications in other fields of research. A knowledge gap exists in the chemistry of non-noble metals, wherein design rules for shape control of NCs are still poorly defined compared to those of other classes of materials. Herein, we demonstrate that tuning the precursor reactivity is crucial to obtaining a continuous shape modulation from single-crystalline to twinned and stacking fault-lined Cu NCs. This tunability is unprecedented for non-noble metal NCs. We achieve this result by using diphenylphosphine in place of the most commonly used trioctylphosphine. Using in situ X-ray absorption spectroscopy, we show that the temperature modifies the reaction kinetics of an in situ-forming copper(I)bromide-diphenylphosphine complex during the synthesis of Cu NCs. We propose the presence of a P-H functionality in the phosphine to explain the higher reactivity of this precursor complex formed with diphenylphosphine compared to that formed with trioctylphosphine. This work inspires future studies on the role of phosphine ligands during the synthesis of Cu NCs to rationally target new morphologies, such as high-index faceted Cu NCs, and can be conceptually translated to other transition-metal NCs.