This letter introduces an analytical model that gives the asymptotic cumulative number of molecules absorbed by spherical receivers in a diffusive multiple-input multiple-output (MIMO) molecular communication (MC) system with pointwise transmitters. In the proposed model, the reciprocal effect among the fully absorbing (FA) receivers is described by using the concept of fictitious pointwise negative source of molecules, the best position of which for each spherical receiver being defined by its absorption barycenter. We show that there is an agreement between the proposed asymptotic model and the numerical solution of the exact analytical model from the literature that describes the interaction among the receivers, which is solved for a sufficient long time. We resort to numerical solution because for the exact model there is no analytical solution apart for the case of one transmitter and two receivers. We demonstrate that the barycenter tends to coincide with the center of the FA receivers in the temporal asymptotic case.