Mallory Swanson is a doubt for the Women’s World Cup after tearing the patella tendon in her left knee.
Swanson, 24, is her country’s leading scorer this year with seven goals in five games.
The World Cup in Australia and New Zealand starts on 20 July.
Reigning champions the US begin their defence against Vietnam in Auckland on 21 July.
Swanson has returned to her NWSL club the Chicago Red Stars for “further evaluation” and no timeline has been put on her recovery.
She has been replaced by Alyssa Thompson in the US squad in preparation for their second friendly against the Republic on 11 April in St Louis.
Victory in the first meeting was secured through goals from Emily Fox and Lindsey Horan, the result ending the Republic’s nine-match unbeaten run as they prepare for their first appearance in a Women’s World Cup.