Chelsea striker Sam Kerr suffered an ACL injury while on a warm-weather training camp in Morocco, the club said on Sunday.
"Sam will be assessed by a specialist in the coming days and then begin her rehabilitation with the club's medical team," Chelsea said in a statement.
Australia's football association said there is no time frame on her return, although it confirmed she is expected to miss the rest of the 2023-24 Women's Super League season.
Kerr is one of the sport's most popular figures and was the first female player to feature on the global cover of the FIFA video game in 2022 alongside men's star Kylian Mbappé.
"Considering how hard Sam has worked over the past six months to return to play [from a calf injury during training at the World Cup], this news is a devastating blow for everyone," Gustavsson said in a statement.
"With her ability to lead by example, Sam's guidance and influence on the team is significant and, as a result, this will be an incredible loss for the national team. Our focus now is on ensuring she has all the support she wants and needs to navigate recovery and rehab."
Kerr has scored 99 goals in 128 matches since joining Chelsea in 2019, helping the club to a number of titles, including four WSL triumphs.
She has also won the league's Golden Boot twice and was runner-up to Spain's Aitana Bonmatí at the Ballon d'Or awards last year.
The injury will mark a big setback to Chelsea as manager Emma Hayes bids farewell this summer to take charge of the United States women's national team. Chelsea are top of the WSL with 25 points after 10 games, three points clear of Arsenal and Manchester City.
Kerr, 30, is the latest women's star to suffer an ACL injury, with a number of major players being forced to miss last summer's Women's World Cup including USWNT forward Catarina Macario, Netherlands' Vivianne Miedema, England's Beth Mead and Leah Williamson.
Global players union FIFPRO said an increased workload, travel and insufficient rest have contributed to an increase in injuries, including torn ACLs, among women's professional players.
In December, European football governing body UEFA introduced a women's health expert panel to seek a deeper understanding of ACL injuries and their occurrence in the women's game.
Information from Reuters contributed to this report.