AC Milan have been told by the USA Football Federation that defender Oguchi Onyewu has sustained a suspected knee tendon injury.
The USA international, who only joined the Serie A giants in the summer, picked up the serious problem while representing his country against Costa Rica on Wednesday.
Onyewu is awaiting the results of further tests, but the 27-year-old fears the worst and could be facing an operation and a long rehabilitation period.
He told skypsorts.com: "It is a big blow for me. I am devastated.
"After a jump, I did not feel my knee. I knew straight away that I was badly injured.
"But that is life. Hopefully, I will come back soon."
After confirming the injury in a short statement on their official website, Milan chief executive Adriano Galliani has announced that his club will seek damages.
"I'm very angry because once again the national teams take our players and we have to pay the consequences of injuries," Galliani is quoted as saying by Italian media.
"We demand compensation from the American Federation because we cannot pay the salary of a player who cannot play for six months."