This work was aimed at evaluating the combined effects of fibre orientation and fibre concentration evolution on rheological predictions by using X-ray computed tomography (X-CT) for morphological analysis. Injection moulded Nylon-6 containing 33% weight short glass fibres (PA6-33GF) were used in this study as they provided a highly reproducible initial fibre orientation and concentration distribution. A cone-plate geometry was designed with a radius and angle of 20mm and 0.1rad respectively and a larger truncation height of 0.8mm for characterising transient simple shear rheology. The larger truncation reduces the contribution from gap effects while imposing a constant shear rate across a larger area of the sample. Rheological measurements were performed with standard silicone oil to establish the accuracy of this geometry. When measuring the transient shear viscosity and first normal stress difference for the PA6-33GF samples, an overshoot attributed to fibre orientation evolution and fibre concentration migration was observed before reaching steady state conditions. Fibre orientation and concentration measurements were taken from the sheared samples at intermediate applied strain values to evaluate the rate of morphological evolution. Fibre orientation evolution and fibre concentration migration were modelled using the Reduced Strain Closure (RSC) model and Suspension Balance Model (SBM) respectively. Using predicted fibre orientation and concentration values, constitutive stress models were tested on their ability to predict the experimentally measured the transient rheological responses. The Evans model coefficients had the best fit to experimental data and showed that fibre concentration migration had a significant effect on shear viscosity predictions while the effect was less pronounced for first normal stress difference predictions. Such a constitutive model is useful for obtaining transient flow predictions for industrial polymer processing applications.