In this work the authors have examined a model-space Brueckner-Hartree-Fock (MBHF) approach to the single-particle energies in nuclear matter, employing three recent versions of the Bonn meson-exchange potential model. The non-relativistic MBHF calculations form the well known ``Coester`` band, where the potential which exhibits the weakest tensor force yields the largest binding energy per nucleon. Correcting for relativistic effects, the MBHF calculations result in too little binding. The implications are discussed. 12 refs., 2 figs.