Onyewu wasn't among the 18 players who dressed for Saturday night's Serie A finale against Juventus. A day earlier, he was one of 22 players initially selected for the game at San Siro stadium.
Onyewu had left knee surgery Oct. 21, seven days after tearing his patellar tendon during a World Cup qualifier against Costa Rica.
He joined AC Milan last summer from Belgium's Standard Liege but played in just competitive match for the rossoneri, appearing as a 60th-minute substitute against Zurich in the Champions League on Sept. 30.
The U.S. team starts workouts Monday at Princeton, N.J. Onyewu could made his first appearance since surgery when the Americans play the Czech Republic on May 25 at East Hartford, Conn.