  The underlying Fermi surface is a key concept for strongly-interacting
electron models and has been introduced to generalize the usual notion of the
Fermi surface to generic (superconducting or insulating) systems. By using
improved correlated wave functions that contain backflow and Jastrow terms, we
examine the two-dimensional $t{-}t^\prime$ Hubbard model and find a non-trivial
renormalization of the topology of the underlying Fermi surface close to the
Mott insulator. Moreover, we observe a sharp crossover region, which arises
from the metal-insulator transition, from a weakly interacting metal at small
coupling to a resonating valence-bond superconductor at intermediate coupling.
A violation of the Luttinger theorem is detected at low hole dopings.
