  The isovector and isoscalar $ND$ $s$-wave scattering lengths are extracted by
fitting to the LHCb data of the $pD^0$ invariant-mass distribution in the decay
$\Lambda_b\rightarrow\pi^-pD^0$, making use of the cusp effect at the $nD^+$
threshold. The analysis is based on a coupled-channel nonrelativistic effective
field theory. We find that the real part of the isovector $ND$ scattering
length is unnaturally large due to the existence of a near-threshold state with
a mass around 2.8 GeV. The state is consistent with the $\Sigma_c(2800)^+$
resonance observed at Belle. Our results suggest that it couples strongly to
the $ND$ channel in an $s$-wave, and that its quantum numbers are $J^P=1/2^-$.
The strong cusp behavior at the $nD^+$ threshold can be verified using updated
LHCb data.
