Poly(lactic acid) (PLA) is a promising bio-based polymer for melt-spun fibres, but its low melt strength, brittleness and narrow processing window limit broader use. This study evaluates PLA/cellulose biocomposites for monofilament melt spinning by comparing micronized bleached Eucalyptus kraft pulp-derived fibres (P200) with microcrystalline cellulose (MCC) as a particulate reference, within a fixed compatibilizer/processing-aid formulation. Composites containing different cellulose contents were prepared by melt compounding and assessed by thermal analysis, oscillatory rheology, mechanical testing and fracture morphology; selected 5 wt.% formulations were then processed into monofilaments. Cellulose incorporation increased the onset degradation temperature of the PLA reference, while the rheological and mechanical responses depended strongly on reinforcement morphology. P200 promoted stronger low-frequency melt structuring, stiffness and crystallization tendency, but also a greater reduction in ductility, with notched impact strength decreasing significantly only at the highest P200 loading. MCC produced a more moderate composite-level response. Stable continuous monofilaments were obtained from both selected cellulose-containing formulations, although neither matched the drawable window or draw-induced mechanical development of the PLA reference formulation. Among the cellulose-containing systems, the 5 wt.% MCC formulation retained the more favourable balance between reinforcement and melt-spinning processability.