A self-administered placebo might still be effective for evolutionary reasons. It would signal that a reduced activity level is related to tending your injuries, rather than, say, waiting in ambush or 'freezing' to avoid notice by motion-sensitive predators, so it's safe to divert resources toward repair or antibody production at the expense of sensory and muscular readiness.
Same reason people have a hard time getting to sleep in unfamiliar circumstances, but focusing on a token reminder of home dispels the feeling.
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