  Starting from a multiband Hamiltonian containing the relevant Ni and O
orbitals, we derive an effective Hamiltonian $H_{eff}$ for the low energy
physics of doped Y$_{2}$BaNiO$_{5}.$ For hole doping, $H_{eff}$ describes O
fermions interacting with S=1 Ni spins in a chain, and cannot be further
reduced to a simple one-band model. Using numerical techniques, we obtain a
dynamical spin structure factor with weight inside the Haldane gap. The nature
of these low-energy excitations is identified and the emerging physical picture
is consistent with most of the experimental information in Y%
$_{2-x}$Ca$_{x}$BaNiO$_{5}$
