Manager 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 (also hamstring) is expected to be out of action for about six weeks.
"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 were all hurt 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 all seven of England's matches at the recent World Cup, but Beard said her injury was not caused by returning to league action too soon following the tournament in Canada.
"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."
Liverpool are the reigning Women's Super League One champions but are currently only sixth in the table after a disappointing start to the season.
With the league concluding in October, Beard admitted that the new injuries would make defending their title even more difficult.
"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."
Beard also told BBC Sport that he did not think he would be in a position to make emergency loan signings to cover for the injured players.

The aye-aye is a kind of lemur, and looks a bit like a cross between a mouse and a bat.
There are as few as 1,000 aye-ayes left in the world.
At one point they were thought to be extinct due to hunting in Madagascar, an island off the southeast coast of Africa.
Lynsey Bugg, from the Zoo said:  "Their mother was very secretive and we didn't realise she'd had twins straight away."
"It's a world first, so it wasn't what we were expecting but it's amazing, really amazing.
"We wanted to monitor them and make sure they were putting on weight before we announced their arrival."
Aye-ayes have been born in captivity in Britain before.
A girl called Kintana was born in 2005, and a boy called Raz was born in 2008.
But this is the first time that twins have been born.
Lynsey said that the twins' mother Sabrina was doing a "great job with her young".
"The twins are physically demanding and it is testament to Sabrina that she and the twins appear to be doing really well," she said.
"Whilst we are keeping a close eye on the group, we try to adopt a hands-off approach and encourage a family dynamic close to one they would experience in the wild."

Selling animals under 8 weeks old has also been stopped under the new rule
Katy Tang, one of the politicians who helped to bring in the ban, said that she did it because she doesn't think some pet stores spend enough time thinking about animals' health or well-being.
She says sometimes puppies and kittens are not treated as well as they should be and animals can also be sold when they are very young - before they are ready to leave their parents.
San Francisco isn't the first place in America to bring a rule like this in.
A number of other cities in America - like Phoenix in Arizona and Montgomery County in Maryland - also have laws which mean cats and dogs being sold in pet shops must come from rescue centres.
In the UK, pet shops can't sell cats or dogs that are under 8 weeks old and the pet shops have to be given a license by the authorities in their area.
Three Liverpool Ladies players, including England midfielder Fara Williams, are set to miss the majority of the club's remaining games this season after picking up injuries.
[NEXT_CONCEPT]
Twin baby aye-ayes have been born in captivity at Bristol Zoo for the first time.
[NEXT_CONCEPT]
San Francisco has brought in a new law saying that pet shops can only sell cats and dogs that come from rescue centres.