  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 $p(X) = A.$ Specifically, we find necessary and
sufficient conditions for $f(p(\lambda))$ to have a factor of degree $n$ over
$\mathbb{Q}.$
