In a previous work, a mechanical model was proposed to predict the reinforcement of amorphous polymers by particulates as well as by unidirectional fibers over wide ranges of volume fractions of fillers and temperatures (or frequencies). This model is based on both the definition of a representative morphological pattern (RMP), accounting for the presence of fiber clusters, and a quantitative morphology analysis, based on the percolation concept. In this work, such an approach is extended to describe the viscoelastic properties of a semicrystalline polymer, poly(butylene terephthalate), commingled with 30 and 50 vol % of unidirectional glass fibers. It is found that aggregates constituted by both fiber clusters and a transcrystalline region (TCR) can act as the continuous phase. Based on the use of a mechanical model in a reverse mode, the actual viscoelastic behavior of this TCR is extracted and compared to that displayed by the unfilled polymer.