Kazhdan-Lusztig left cell preorder and dominance order

Let "$\leq_L$" be the Kazhdan-Lusztig left cell preorder on the symmetric group $S_n$. Let $w\mapsto (P(w),Q(w))$ be the Robinson-Schensted-Knuth correspondence between $S_n$ and the set of standard tableaux with the same shapes. We prove that for any $x,y\in S_n$, $x\leq_L y$ only if $Q(y)\unrhd Q(x)$, where "$\unrhd$" is the dominance (partial) order between standard tableaux. As a byproduct, we generalize an earlier result of Geck by showing that each Kazhdan-Lusztig basis element $C'_w$ can be expressed as a linear combination of some $m_{uv}$ which satisfies that $u\unrhd P(w)^*$, $v\unrhd Q(w)^*$, where $t^*$ denotes the conjugate of $t$ for each standard tableau $t$, $\{m_{st} \mid s,t\in Std(\lambda),\lambda\vdash n\}$ is the Murphy basis of the Iwahori-Hecke algebra $H_{v}(S_n)$ associated to $S_n$.