  We study the evolution of a Mott-Hubbard insulator into a correlated metal
upon doping in the two-dimensional Hubbard model using the Cellular Dynamical
Mean Field Theory. Short-range spin correlations create two additional bands
apart from the familiar Hubbard bands in the spectral function. Even a tiny
doping into this insulator causes a jump of the Fermi energy to one of these
additional bands and an immediate momentum dependent suppression of the
spectral weight at this Fermi energy. The pseudogap is closely tied to the
existence of these bands. This suggests a strong-coupling mechanism that arises
from short-range spin correlations and large scattering rates for the pseudogap
phenomenon seen in several cuprates.
