The off the ball incident occurred 38 minutes into Inter's defeat against Chievo.
The two players had argued a few moments earlier and Eto'o opted to take 'revenge' by running towards him to head-butt Cesar in the chest.
The referee, assistants and fourth official at the Stadio Bentegodi did not see the incident, but it was caught on camera.
This means Eto'o should prepare for video evidence to be used against him and potentially a lengthy ban.
Inter coach Rafa Benitez said: “You can see he got punched and was provoked. He reacted this way and lost control.
“It’s a shame, but there are four officials on the field and if they didn’t consider it to be an incident worth acting on, then clearly that’s their opinion.”