Megan Rapinoe suffered a torn ACL in her right knee during training with the United States on Friday.
Rapinoe's injury was confirmed by an MRI scan after she was hurt in Honolulu, but no time frame was given for her recovery.
"I want to thank my teammates and the staff for the amazing support I've received so far," Rapinoe said in a statement. "I know I'll be in the best of hands moving forward and I'll be doing everything I can to get healthy as soon as possible.
"This is obviously a very difficult time, but I do feel super confident in my recovery as I've gone through this before and I know what the process will be like. I want everyone to know that I'll be fine, and that I'm currently accepting any recommendations for TV shows to binge watch and great restaurants that deliver in Seattle. I'm also going to get really good at FIFA 16."
Rapinoe could miss the Olympics next summer.