Van Persie was struck in the back of the head from close range by a Williams clearance after being fouled by Nathan Dyer on the edge of the Swansea box in the 75th minute.
The Wales international did not appear to be trying to strike Van Persie, although the whistle had gone.
The Dutchman reacted angrily, sparking a melee which resulted in both players receiving a booking from referee Michael Oliver.
Ferguson launched a furious attack on Williams, accusing the centre-half of deliberately trying to hurt Van Persie, and suggested the striker could have suffered a broken neck.
"The Van Persie situation you can clearly see that he could have been killed," Ferguson said on Sky Sports News.
"The FA has got to look into it regardless that he has been given a yellow card. He should be banned for a long time because that was the most dangerous thing I've seen on a football field for many years.
"It was absolutely deliberate. The whistle has gone, the game has stopped and he has done that right in front of the referee, he could have killed the lad.
"It was a disgraceful act by the player, he should be banned for a long time."