Sarkisian initially was placed on an indefinite leave of absence Sunday after athletic director Pat Haden learned he had showed up for a team meeting seemingly intoxicated, but after a further investigation into the coach's conduct, Haden determined Sarkisian "was not healthy" and made the decision to fire him.
Sarkisian has since entered an out-of-state treatment facility, a source told ESPN's Joe Schad.
After practice Wednesday, USC quarterback Cody Kessler voiced his support for his former coach.
"Obviously, [Sarkisian is] dealing with some tough stuff right now, but I wish him nothing but the best. I really do," Kessler said. "There's no hard feelings from me. I've been praying for him. I've been hoping things are going to go good for him. He's always been great to me, so I have no hard feelings there.
"I know this team doesn't, either. The first thing this team wants is for him to get better. We're still going to support him as he goes through the next steps he takes, but our main focus right now is getting back out there. We still have a game Saturday."
Sarkisian's trouble with alcohol drew national attention in August after he appeared under the influence at a booster event and used profanity while addressing a crowd of an estimated 2,500 people.
He later apologized for his behavior and blamed it on using a mixture of medicine and alcohol, and said he would seek unspecified treatment.
The Trojans' offensive coordinator, Clay Helton, was named interim head coach and will lead the team for the rest of the season.
Trojans looking to put noise behind them
Ted Miller and Brad Edwards preview USC's trip to Notre Dame, where the Trojans will once again be looking to put a midseason firing behind them and rally.