England and Arsenal playmaker Jordan Nobbs could miss the rest of the season and is a doubt for next summer's Women's World Cup after rupturing her anterior cruciate ligament.
The Gunners say the 25-year-old is "receiving specialist care" after suffering the injury in Sunday's win at Everton in the Women's Super League.
Nobbs, who has 56 senior international caps, was England's player of 2016.
She has been a key part of the Arsenal side that have a 100% record this term.
The Gunners are top of the WSL table and in-form Nobbs has eight league goals in eight league appearances this season.
The Women's World Cup - at which a fully-fit Nobbs would be expected to start for Phil Neville's Lionesses - begins on 7 June 2019 in France.
An Arsenal statement on Monday said: "Following scans this morning, we can confirm that Jordan has suffered an ACL rupture in her left knee and she is now receiving specialist care.
"Everyone at the club wishes Jordan a speedy recovery and will give their full support in her journey back to full fitness."