Johnson was spitting up blood after the incident and was taken to a nearby hospital for surgery, the Los Angeles Times reported.
"It was an unfortunate accident, and Stafon is getting great care right now," USC head Coach Pete Carroll said. "We don't have a lot of information at this point, and we'll keep everyone updated, but our thoughts and prayers are with Stafon."
Johnson, a senior, has rushed for 157 yards and a team-leading five touchdowns on 32 carries this season.
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