USC officially removed tailback Anthony Brown from its roster.
A USC spokesperson said Brown's name was taken off the Trojans website roster last Thursday evening.
Brown did not respond to text messages asking if he wrote the post, which has since been removed.
Brown played cornerback and started six games during his first four seasons at USC. He requested and was granted a move to tailback after Sarkisian was hired in December.
Brown did not participate in players-only summer workouts. He appeared sharp early in training camp but was then sidelined because of injuries.
Sarkisian said he met with Brown and encouraged him to continue with the team. The post to Brown's Instagram account appeared later.
Sarkisian said the allegations were "ridiculous."