Among nanocellulose materials, MFC has recently been used as a reinforcing filler in petrochemical thermoplastics [26]. The use of MFC in green composites can be particularly advantageous in order to achieve significant improvements in the mechanical and oxygen barrier performance, which are the most limiting factors of biopolymers. However, when ultrafine cellulose structures are used as a mixing element to reinforce polymer matrices, there are two main technological challenges to consider. On the one hand, ultrafine cellulose structures are mostly prepared in the form of aqueous suspensions that are usually not practical in terms of processability and industrial use. During the drying or dewatering process, these tend to form aggregates due to the high hydrophilic nature of cellulose, which has a surface that is very rich in hydroxyl groups (-OH) that are prone to form strong intermolecular hydrogen bonds. This effect is amplified in the case of nanostructured materials due to its large surface-to-volume aspect ratio and high surface energy [27]. For example, it has been observed that during the spray-drying process, cellulose nanoparticles agglomerate by a capillarity effect and the formation of hydrogen bonds and van der Waals forces [28]. Alternatively, lyophilization or freeze drying led to ice crystal growth, which tends to favor the agglomeration of nanocelluloses [29]. On the other hand, the presence of a large number of -OH groups on the cellulose surface results in a polar fiber surface with a relatively low compatibility with the polymer matrices. This habitually results in a non-uniform dispersion and distribution of microfibers during the melt-transformation processes by which nanocelluloses are incorporated into polymers, which leads to a significant loss of performance in the composites [30]. Different strategies have been carried out in order to improve the interfacial adhesion of green composites based on MFC or CNC, mainly by means of chemical pre-treatments and compatibilizing agents or coupling agents [31]. For example, some studies have focused on modifications on the surface of the cellulose through chemical or physical pre-treatments to reduce its hydrophilicity [32]. However, although these methods can positively result in a decrease in moisture absorption and an increase in the mechanical properties, they are usually expensive and involve the use of additional processing steps or toxic chemicals that could be an impediment to the subsequent use of the resulting composites [27]. Other recent studies have reported the use of compatibilizers such as polymers grafted with maleic anhydride (MAH) [33] or multi-functional additives based on epoxy groups [34] and isocyanates [35]. This research reports a novel and potentially scalable technique to prepare green composites of PHBV and MFC based on two steps. Initially, an aqueous MFC suspension was atomized to yield micron-sized droplets containing the dry cellulose microstructures that evaporated upon deposition, leaving no water residues. Secondly, the resultant spray-dried cellulose ultrathin structures were incorporated at different contents, that is, 2.5, 5, and 10 wt %, into PHBV by melt-mixing and their processing and properties were analyzed. Finally, the green composite with the optimal MFC content was compatibilized with two different reactive additives in order to enhance the performance of the films for potential uses in food packaging applications.