In the paper in quantum fields theory by Gribov,V.; (1978) "Quantization of non-Abelian gauge theories". Nuclear Physics B. 139: 1–19; in Section 3 the author makes the following claim from PDE and operators theory without any explanation which I would like to understand.

Let $\frak{g}$ be a Lie algebra of a compact Lie group. (If you feel uncomfortable with general Lie algebras you may think of a special case $\frak{g}=\mathbb{R}^3$ with the operation of Lie bracket $[\cdot,\cdot]$ equal to the vector product $\times $.) For $\mu=1,\dots,4$ let $$A_\mu\colon \mathbb{R}^4\to \frak{g}$$ be fixed smooth functions with compact support and satisfying $\partial_\mu A_\mu=0$ (where there is a summation convention in repeated indexes).

Consider the differential operator $L$ on $\frak{g}$-valued functions $$L\alpha =-\Delta \alpha +[A_\mu,\partial_\mu \alpha],$$ where $\Delta$ is the ordinary Laplacian acting component-wise, $[\cdot,\cdot]$ is the Lie bracket. Clearly this is a symmetric operator.

As far as I understand, Gribov claims that $L$ has no negative discrete spectrum provided $A_\mu$ are very small (in some sense). Why??