The rheology of highly concentrated fibers suspended in power-law fluids is investigated by upscaling the physics at the fiber scale. A deterministic upscaling technique is used, namely the homogenization method for periodic discrete structures. This micro-macro approach is used to carry out a quantitative study of concentrated fiber suspensions with planar fiber orientation, performing "numerical rheometry experiments" on a set of representative elementary volumes of fiber suspensions. The simulations underline the significant influence of the fiber volume fraction and orientation, as well as of the non-Newtonian properties of the suspending fluid on the resulting macroscopic rheological behavior. The predictions of the model are compared with experimental results obtained on an industrial thermoset short fiber-bundle polymer composite (SMC).