Although Plasencia was licensed to prescribe and administer the potent tranquilizer, prosecutors allege that he exploited the Hollywood star's addiction for personal gain.
Luna served in the US Air Force from 2009 to 2014 and also worked as a model, appearing on Sports Illustrated's website in 2013 and as a swimsuit model in Maxim magazine in 2014.
The sheriff's office launched an investigation, and detectives found that the preteen sustained "serious life-threatening" injuries on Thursday evening.