The 33-year-old Paraguayan international suffered knee ligament damage in Sunday's FIFA World Cup qualifier against Venezuela. He limped off in the 80th minute having headed the only goal of the game. Gamarra had just re-signed for Inter on a one-year contract.
The centre-back, who won a silver medal at the Athens Olympics, is expected to undergo more medical tests but Paraguay team doctor Osvaldo Pangrazio said he would be sidelined for at least three weeks. That would rule him out of next Tuesday's Group G home match against Werder Bremen and the fixture at RSC Anderlecht on 29 September.
Inter have made several defensive changes this summer, most notably with Fabio Cannavaro leaving for Juventus FC. The Milan club have acquired Sinisa Mihajlovic, Zé Maria and Nicolás Burdisso. Roberto Mancini's team start their Serie A campaign at AC Chievo Verona on Saturday night.
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