Galois groups and rational solutions of $p(X)=A$

We extend Theorem 1 of R. Reams, A Galois approach to m-th roots of matrices with rational entries, LAA 258 (1997), 187-194. Let $p(\lambda )$ be any polynomial over $\mathbb{Q}$ and let $A\in M_n(\mathbb{Q})$ have irreducible characteristic polynomial $f(\lambda )$ with degree n. We provide necessary and sufficient conditions for the existence of a solution $X\in M_n(\mathbb{Q})$ of the polynomial matrix equation $p(X)=A\mathrm{.}$ Specifically, we find necessary and sufficient conditions for $f(p(\lambda ))$ to have a factor of degree $n$ over $\mathbb{Q}\mathrm{.}$