Poly (butylene terephthalate) (PBT) nanofibrils (NFs) were developed in-situ by implementing a rheologically tuned melt spinning process on poly (lactic acid) (PLA)/PBT blends. In the absence of any flow fields, melt crystallization of fabricated PLA/PBT in-situ nanofibrillar composites (NFCs) resulted in orthogonal patterning of PLA nanodomain crystallites around single or double PBT NFs. Geometric confinement (soft epitaxy) was the nucleation mechanism of PLA chains at the interface of isotropic distributed PBT NFs followed by the formation of reprocessible 2D nanohybrid shish-kebab (NHSK) superstructures. By increasing the PBT NFs content and their bundling, a fan-shaped transcrystalline structure was formed due to seeding or epitaxial nucleation on the bundle of PBT NFs owning a less curved interface. The nucleation and growth mechanism of crystallites structures which were influenced by temperature and PBT NF content have been discussed thermodynamically and kinetically by parallel investigations on microscopic observations and X-ray diffraction analysis. The evolution of different crystalline structures in quiescent melt in the forms of spherulites, NHSK, or fan-shaped transcrystallinity induces a local orientation in the isotropic NFCs. Mechanical propertie improvements of NFCs were compared in amorphous and crystalline states with tailored NHSK superstructures. Display omitted •Geometric confinement (soft epitaxy) was the nucleation mechanism in the formation of reprocessible 2D nanohybrid shish-kebab (NHSK) superstructures.•The PLA fan-shaped transcrystalline structures were formed by increasing the PBT NFs content and their bundling due to seeding or epitaxial nucleation on the bundle of PBT NFs.•The curvature of the interface in fibrillar composites plays a dominant role in crystallite nucleation and final crystalline phase structure.•The evolution of crystalline structure in the form of spherulites, NHSK, or fan-shaped transcrystalliny induces a local orientation in isotropic NFCs.•The crystalline phase morphology of PLA was tailored to improve its mechanical properties while stopping aging.