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 $\mathbb{ Q}.$
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