We investigate the impacts of resonant and non-resonant backgrounds to $B\to K^\ast(\to K\pi)\nu\bar{\nu}$ decay.
Such effects arise beyond the narrow width approximation of the $K^\ast$ meson. The non-resonant amplitudes are studied
using Heavy-Hadron-Chiral-Perturbation theory
in the kinematic region of low hadronic recoil. We find that in the $K^\ast$ signal window
the non-resonant amplitudes induce an uncertainty of about 20% in the branching fraction, and
at most few % in the longitudinal polarization fraction $F_L$. Uncertainties induced by broad scalar resonances $K^\ast_0$ and $\kappa$
are at the level of few percents in the branching fraction in the $K^\ast$ signal window and negligible in longitudinal polarization fraction $F_L$.
Since the effects of background in $F_L$ are small, this observable can be used to test form factors, or alternatively
the right-handed currents in the entire $q^2$-region. We define a new observable, the forward-backward asymmetry $A^K_{\rm FB L}$
that can be used to experimentally constrain the resonant and non-resonant backgrounds.