Manchester City midfielder Jill Roord has suffered a rupture to her anterior cruciate knee ligament.
She suffered the injury during City's League Cup win against Manchester United on Wednesday.
She has 98 caps for the Netherlands and has scored six goals and registered two assists for City in the league this season.
"Jill will remain under the care of City's medical team and will begin her rehabilitation," the club said.
"Everyone at City wishes her a full and speedy recovery."
Emma Sanders, BBC Sport
Eyebrows were raised when Manchester City's summer transfer window passed with just one arrival - as Roord came in for a club record fee.
While title rivals Arsenal and Chelsea shopped more fruitfully, City saw Roord as the player who could bring everything together as they strived for continuity.
Her impact was instant.
With six goals and two assists in 11 games, she has been one of the WSL's outstanding players this season, adding spark and creativity to an already frightening Manchester City attack.
Her loss will be felt considerably in the second half of the season as City chase Chelsea for the WSL title - and with an expected recovery time of at least nine months, she is likely to miss the start of next season too.
Manager Gareth Taylor could dip into the January transfer window for a late replacement but only has until Wednesday's deadline to do so.