1. Fundamental entities evolve into more complex entities. (Premise)
2. If the PSR is false, evolution is most likely inexplicable. (Premise)
3. Evolution is explicable. (Premise)
4. Therefore, the truth of evolution implies that the PSR is most likely true. (From 1 - 3)
Since a denial of evolution leads to a conclusion incompatible with naturalism, the naturalist will almost certainly accept (1). In fact, it's hard to think of any contemporary naturalists who would reject the first premise. (3) is true on any realistic account of Darwinian natural selection. This leaves us with premise (2).
Take an inductive form of the PSR: if X exists, then X most likely has an explanation of its existence. If most things do not have explanations, then evolution prima facie has an explicability likelihood of <.5. However, evolution is explicable according to (3). This entails that the PSR is most likely true prima facie.