The backup running back was placed on the Reserve/Did Not Report list Saturday after failing to show for the start of training camp.
Mason did not participate in Los Angeles' offseason program. Coach Jeff Fisher attributed his absence to "personal reasons."
Fisher revealed Saturday that Mason has broken off communications with the team:
TMZ Sports reported earlier this week that Mason's family is "extremely concerned" about his mental health, citing a "string of bizarre behavior and angry outbursts."