Foster says he fears a goalkeeper could be knifed by a crazed fan on the pitch.
He said: "That lad could have had a blade. It is ridiculous, if you think of it like that. It is a bit scary, to be honest.
"I was watching it and when that happened it made me feel sick. It was horrible."
Foster believes it is time for heavy penalties, adding: "The guy who interrupted the boat race got six months. Well, if he gets six months, why can't someone who runs on to the pitch and slaps Chris Kirkland in the face get at least that?"