With fans on the field at Corvallis, Oregon, following No. 21 Oregon State's 38-34 win, a video posted on social media shows Johnson walking on the turf when a Beavers fan wearing a hat appears to hold up his hand to Johnson while crossing his path.
The fan lowered his hand and continued walking away, and the video then shows Johnson extending his right hand forcefully. The fan appeared to be struck by Johnson, with footage showing his hat falling to the ground and Johnson being quickly pulled back by two individuals wearing Ducks gear.
"We are aware of a postgame exchange involving a member of our football team and a fan and are working to gather more information," Oregon said in a statement.
Johnson had three tackles in Saturday's game. In 11 games this season, he has a team-best 6.0 sacks.