An ordered copper-phthalocyanine (CuPc) single layer deposited on the Au(110) surface has been electron-doped by exposition to potassium. Progressive occupation of the former lowest unoccupied molecular orbitals (LUMOs) up to the filling of the twofold degenerate LUMO and $\text{LUMO}+1$ and $\text{LUMO}+2$ levels, has been detected by near-edge x-ray absorption spectroscopy spectra. The electronic states filled by electron transfer from the alkali atoms are localized at the interface, and the process induces a redistribution of the density of states with a conspicuous decrease in spectral density at the Fermi level. Localization of the final states, sudden decrease in density of states at the Fermi level, minor screening of the electron-hole excitations, and enhanced excitonic effects of doped CuPc with respect to the pristine CuPc single-layer concur to suggest that the electron injection produces a Mott insulator where correlation effects dominate the electronic properties.