Gordon did not practice Friday and has been ruled out for Sunday's game with a hip strain and knee sprain. He will finish the season three rushing yards short of 1,000.
Chargers head coach Mike McCoy said he and general manager Tom Telesco delivered the news to a disappointed Gordon before practice Friday.
"He was crushed," McCoy said. "It's what you thought his reaction would be, just because he's worked so hard. I wish everybody worked as hard as he did to get back to where he was today."
McCoy went on to say that Gordon understood the situation.
A Boston College product, Williams will be active for the first time this season after spending most of the year on the practice squad.
"I'm just excited to get on the field and get some action with these guys," Williams said. "I've been watching them all year and I see how much talent they have on this team. It's exciting to watch."
Williams said he knows the offense but has not had the on-field experience working with Philip Rivers and the rest of the offense, so that will be an adjustment Sunday.
"I'm just going to go out and play the game and have a lot of fun doing it," Williams said.