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Electron-doping FeSe stabilizes its superconducting state, raising T_c from around 8 K in the bulk to above 30 K. Monolayer (ML) FeSe on SrTiO₃ (and other transition-metal-oxides) shows a similar or greater enhancement to T_c, accompanied by the vanishing of a Γ-centered hole pocket in the electronic band structure obtained from ARPES. Earlier work has proposed that O vacancies or excess Ti at the FeSe/STO interface could be responsible for this doping. We present calculations based on density functional theory that model the electronic structure of FeSe in the presence of disordered charge-doping impurities in the underlying Ti-O layer. This Wannier-based method allows one to disentangle the underlying cause of the doping from specifics of a given substrate-ML interface. ML FeSe behaves similarly on TiO₂ and LaTiO₃ as well as on substrates not containing Ti-O_x layers (e.g. FeSe/LaFeO₃). We study disordered defects (interstitial cations and/or oxygen vacancies) at the FeSe / STO interface and determine their effect on the averaged-out Fermi surfaces measured by ARPES, providing direct insight into the connection between the atomic structure of this interface and the observed electronic properties that coincide with the superconducting state.