We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wavevector dependence of quasielastic scattering {\em across} the phase transition. Temperature scans at a small energy transfer of $0.5$~meV establish an abrupt step-like suppression of the excitations at $\vec{Q}_1=\qone$ below $T_0$, whereas excitations at $\vec{Q}_0=\qzero$, associated with large-moment antiferromagnetism (LMAF) under pressure, are enhanced with a pronounced peak {\em at} $T_0$. This is the behavior expected of a ``super-vector'' order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.