On Thursday, Gibson told The New York Post in a statement, "There were creative differences on the set and a disagreement. I regret that it occurred. We all want to work together as a team to make the best show possible. We always have and always will."
But a source close to Gibson insisted that the incident wasn't the actor's fault.
"[Williams] got very aggressive, so Thomas walked away and then he felt like the guy was coming after him," the source told us. "He turned around and kicked just on instinct, like a reflex."
He has been on the show since 2005 and has known Williams for most of that period.
Gibson, who made his big break in the show Dharma & Greg, had previous onset outbursts, TMZ reported, including one in which he pushed an assistant director and was subsequently sent to anger management courses.
A rep for Gibson didn't immediately respond to a request for comment.