Cellulose nanocrystals (CNCs) have attracted much interest as a nucleating and reinforcing agent for poly-L-(lactid acid) (PLLA). However, the processing route to incorporate CNCs into PLLA matrices and the CNCs content have a strong impact on the nucleating and reinforcing efficiency of the cellulose nanofillers (CNs). In the present work, a single melt-processing mode using poly(ethylene glycol) (PEG) as carrier for the CNCs and aminated CNCs (Am-CNCs), prepared via periodate oxidation followed by the addition of diaminododecane, was adopted to elaborate plasticized PLLA composites with different CN contents. The nucleating efficiency was investigated by differential scanning calorimetry using Avrami's approach to determine the half-crystallization time (t1/2). The influence of the CNCs content on the mechanical properties and melt rheology was also assessed by tensile test and frequency sweep measurements. The incorporation of CNCs and Am-CNCs resulted in a marked enhancement in both tensile modulus and strength, with an optimal effect at 2% nanofiller content, and the best performance for the Am-CNCs. A similar tendency was also observed for the melt rheology of the PLLA/PEG blend, which demonstrated a significant increase in both G and G with respect to the neat PLLA/PEG matrix. The results were justified by the effective nucleating ability of the CNCs and their dispersion degree within the plasticized PLLA matrix.