A 28-year-old woman is brought to the trauma centre following a stab wound to the right upper abdomen. Diagnostic laparotomy reveals a Grade IV hepatic injury with a large stellate laceration and active venous bleeding from the hepatic parenchyma. After initial resuscitation and temporary control of bleeding with packing, the surgeon decides to manage the injury non-operatively with planned re-exploration. Which agent is the drug of choice for thromboprophylaxis in this patient during the recovery phase?