Definition of Phase Noise
I think I resolved my confusion, I am posting how I understand it as an answer:

\$ L(f) = \frac{S_\phi(f)}{2}\$;
\$ S_\phi(f)\$ : One-sided spectral density of the phase deviation
\$L(f)\$: One-half of the phase instability 

From the RF spectrum, one can compute \$L(f)\$ by computing noise power around the carrier in dbc/hz giving \$\sin(\phi)\$. Since \$\phi\$ is small, \$\sin(\phi) = \phi\$, you got the spectrum of phase noise. So one does not need to compute phase deviations to compute phase noise.