AC Milan defender Oguchi Onyewu will undergo tests in the coming days to determine the extent of his knee injury.
Onyewu hurt his left knee playing for the United States during Wednesday night's World Cup qualifier against Costa Rica.
A statement from AC Milan read: "The US football federation have informed us of Oguchi Onyewu's suspected tendon rupture of his left knee. The player, who is in the United States, will undergo further tests and if the diagnosis is confirmed, he will have surgery.
"Milan doctor Massimo Manara is in constant contact with Onyewu.''
U.S. coach Bob Bradley said the defender would be out for up to four months after tearing a patellar tendon in his left knee.
"I'm very angry because once again the national teams take our players and we have to pay the consequences of injuries," Milan chief executive Adriano Galliani was quoted as saying by Italian media on Thursday.
"We demand compensation from the American Federation because we cannot pay the salary of a player who cannot play for six months."
Onyewu landed badly after jumping for a header, immediately called for assistance and was taken off the field on a stretcher.
The player has hardly featured at Milan since joining on a free transfer from Standard Liege in the close season.