An improved self-consistent scheme accounting for the 2D geometric arrangement of fibres within the matrix is proposed to predict the influence of the reinforcement effect of DGEBA/anhydride matrix by unidirectional raw glass fibres or coated fibres with silicone or DGEBA/silane coupling agents. This model is based on both (i) an analytical morphology approach underlying the percolation concept and (ii) the definition of an original 'representative morphological pattern' accounting for local phase inversions owing the heterogeneous morphology of composite materials. It is shown that the nature of fibre sizing can influence the viscoelasticity of composite materials by inducing variations in morphology and then in the mechanical coupling. This could result from the more or less good wettability of fibres by the resin during the process. After removing the reinforcement effect through such a theoretical approach, microstructural changes of the polymer matrix induced by fibres, i.e a decrease in the crosslinking degree of the epoxy network, are quantified but no specific influence of the nature of the interface can be revealed.