  Let $G$ be a semi-simple Lie group in the Harish-Chandra class with maximal
compact subgroup $K$. Let $\Omega_K$ be minus the radial Casimir operator. Let
$\frac{1}{4} \dim(G/K) < S_G < \frac{1}{2} \dim(G/K) , s \in (0, S_G]$ and $p
\in (1,\infty)$ be such that \[ \left| \frac{1}{p} - \frac{1}{2} \right| <
\frac{s}{2 S_G}. \] Then, there exists a constant $C_{G,s,p} >0$ such that for
every $m \in L^\infty(G) \cap L^2(G)$ bi-$K$-invariant with $m \in {\rm
Dom}(\Omega_K^s)$ and $\Omega_K^s(m) \in L^{2S_G/s}(G)$ we have, \[ \Vert T_m:
L^p(\widehat{G}) \rightarrow L^p(\widehat{G}) \Vert \leq C_{G, s,p} \Vert
\Omega_K^s(m) \Vert_{L^{2S_G/s}(G)}, \] where $T_m$ is the Fourier multiplier
with symbol $m$ acting on the non-commutative $L^p$-space of the group von
Neumann algebra of $G$. This gives new examples of $L^p$-Fourier multipliers
with decay rates becoming slower when $p$ approximates $2$.
