  The large-scale diffuse $\gamma-$ray flux observed by Fermi-LAT in the 1-100
GeV energy range, parameterized as $\propto E^{-\Gamma}$, has a spectral index
$\Gamma$ that depends on the distance from the Galactic center. This feature,
if attributed to the diffuse emission produced by cosmic rays (CR) interactions
with the interstellar gas, can be interpreted as the evidence of a progressive
CR spectral hardening towards the Galactic center. This interpretation
challenges the paradigm of uniform cosmic rays diffusion throughout the Galaxy.
We report on the implications of TeV Pulsar Wind Nebulae observed by the HESS
Galactic Plane Survey in the 1-100 TeV energy range for the interpretation of
Fermi-LAT data. We argue that a relevant fraction of this population cannot be
resolved by Fermi-LAT in the GeV domain providing a relevant contribution to
the large-scale diffuse emission, viz. $\sim 30\%$ of the total diffuse
$\gamma$-ray emission in the inner Galaxy. This additional component naturally
accounts for a large part of the spectral index variation observed by
Fermi-LAT, weakening the evidence of CR spectral hardening in the inner Galaxy.
