Using the Nambu-Jona-Lasinio model to describe the nucleon as a quark-diquark state, we discuss the stability of nuclear matter in a hybrid model for the ground state at finite nucleon density. It is shown that a simple extension of the model to simulate the effects of confinement leads to a scalar polarizability of the nucleon. This, in turn, leads to a less attractive effective interaction between the nucleons, helping to achieve saturation of the nuclear matter ground state. It is also pointed out that that the same effect naturally leads to a suppression of "Z-graph" contributions with increasing scalar potential.PACS: 12.39 Fe (chiral Lagrangians), 12.39 Ki (relativistic quark model), 21.65+f (nuclear matter), 24.85+p (quarks, gluons and QCD in nuclei and nuclear processes). Keywords: quark-diquark structure of the nucleon, nuclear matter stability, Nambu-Jona-Lasinio model for quarks.