  We employ a manifestly covariant formalism to compute the tree-level
amputated Green's function of non-minimally coupled scalar fields in quadratic
gravity in a de Sitter background. We study this Green's function in the
adiabatic limit, and construct the classical Newtonian potential. At short
distances, the flat-spacetime Yukawa potential is reproduced, while the
curvature gives rise to corrections to the potential at large distances. Beyond
the Hubble radius, the potential vanishes identically, in agreement with the
causal structure of de Sitter spacetime. For sub-Hubble distances, we
investigate whether the modifications to the potential reproduce Modified
Newtonian Dynamics.
