The decay of the narrow resonance $overline{B}{}_{s2}^{*0}!rightarrow B^- K^+$ can be used to determine the $B^-$ momentum in partially reconstructed decays without any assumptions on the decay products of the $B^-$ meson. This technique is employed for the first time to distinguish contributions from $D^0$, $D^{*0}$, and higher-mass charmed states ($D^{**0}$) in semileptonic $B^-$ decays by using the missing-mass distribution. The measurement is performed using a data sample corresponding to an integrated luminosity of 3.0 fb${}^{-1}$ collected with the LHCb detector in $pp$ collisions at center-of-mass energies of 7 and 8 TeV. The resulting branching fractions relative to the inclusive $B^- !rightarrow D^0 X mu^- overline{ u}_mu$ are $f_{D^0} = mathcal{B}( B^- rightarrow D^0mu^-overline{ u}_mu )/mathcal{B}( B^- rightarrow D^0 X mu^- overline{ u}_mu ) = 0.25 pm 0.06$, $f_{D^{**0}} = mathcal{B}( B^- rightarrow ( D^{**0} rightarrow D^0 X)mu^-overline{ u}_mu )/mathcal{B}( B^- rightarrow D^0 X mu^- overline{ u}_mu ) = 0.21 pm 0.07$, with $f_{D^{*0}} = 1 - f_{D^0} - f_{D^{**0}}$ making up the remainder.
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