Fergie fumes over Williams clearance
The defender's clearance hit a prone van Persie at point-blank range during United's 1-1 draw in south Wales, prompting an angry response from the Dutchman.
Williams - booked at the time - afterwards insisted it was a complete accident, but a simmering Ferguson claimed van Persie could have been killed and called the act "absolutely deliberate."
He told Sky Sports: "I think the FA has got to look into it. Irrespective of him having a yellow card he should be banned for a long, long time. It's the most dangerous thing I've seen on a football field for many, many years.
"(It was) absolutely deliberate. the whistle's gone, the game has stopped and he's done that a foot from the player. He could have killed the lad - it's a disgraceful act and he should be banned for a long, long time."
Williams said: "I tried to clear the ball it hit him on the head. I was trying to clear the ball. He was obviously a bit angry about that. It was a much ado about nothing."