Coyotes defenseman Ed Jovanovski was spinning off a check of Kings center Anze Kopitar to hit Williams behind the Phoenix net when both players fell onto Williams.
Williams fell backward with his right leg trapped underneath his torso, dropped onto his back and remained on the ice as play continued briefly in the Kings' end. He was taken off by stretcher after a lengthy delay at 4:27 of the first period.
"It's not good," Kings coach Terry Murray said. "We'll take him back on the flight with us tonight, have the team doctors look at him and get the word out."
Williams has eight goals and 16 assists in 33 games.