Onyewu was injured in last Wednesday's FIFA World Cup™ qualifier against Costa Rica in Washington, which ended in a 2-2 draw. USA had already qualified for the 2010 World Cup finals in South Africa going into that match.
Onyewu, 27, left Belgian club Standard Liege after last season. He inked a three-year contract with AC Milan but appeared in just one of the club's 10 competitive matches this season before he was hurt.
US team doctor Bert Mandelbaum is to perform the surgery in Los Angeles. The timeframe for Onyewu's recovery will be determined after the operation.