The Engineered Cementitious Composite (ECC) model has found that ≤ 2% by volume of discontinuous fiber is favorable for strain-hardening behavior and enhanced ductility in commercial concrete. Using this model, we attempted to mechanically optimize injectable calcium phosphate bone grafts via fiber reinforcement of cellulose nanocrystals (CNC) and gelatin polymer fibers. Results displayed significant decreases in the bone grafts’ hardness after the 1–2% CNC critical point (p < 0.001) due to fibril agglomeration, which likely prompted internal cracking and lowered the samples’ tolerance for plastic deformation. In support, Fourier transform infrared (FTIR) spectroscopy displayed decreased levels of cellulose-gelatin hydrogen bonding after this critical point due to decreases in transmittance broadening. Scanning electron microscopy (SEM) also showed that low-fiber CNC samples exhibited extensive microcracking compared to higher-fiber CNC samples, which contained larger cracks and fiber aggregates, indicating catastrophic failure and reduced hydrogen bonding between cellulose and gelatin.