Reds boss Matt Beard told BBC Sport that Williams and Nigeria striker Asisat Oshoala would miss "a couple of months" with hamstring and knee problems.
Defender Satara Murray is expected to be out of action for about six weeks with a hamstring injury.
"It's a big blow," said Beard. "It's the spine of your team – your central midfielder, your centre-back and your centre-forward – but it's one of those things."
The players all suffered their injuries during Saturday's WSL 1 defeat by Notts County and did not feature in Thursday's 2-0 Continental Tyres Cup victory against Everton.
Williams played in every game for England at the World Cup
Williams played in all seven of England's matches as they finished third at the recent World Cup, but Beard said her injury was not caused by returning to league action too soon following the showpiece.
"It was an impact injury," he said. "Unfortunately as she's tried to win the ball, her leg's completely hyperextended, which has caused the injury."
With the FA WSL 1 concluding in October, Beard admitted that the new injuries would make defending their title even more difficult, with his side currently sixth in the table.
"They're important players to us but it's going to give maybe one or two of the younger players an opportunity," he continued.
"We'll do as much as we can, while we're still involved in everything mathematically, to finish as high as we can."
