"I feel fine, there have been no complications, nothing that has restricted me from any training activities or game activities to present so I feel good," Onyewu told reporters at the team's countryside base near Pretoria on Tuesday.
"I am physically fit, I have had to do all the physical work with the team and even before that I had to do a fitness test in Milan in order to be considered for match fitness - there are no issues in regard to me being able to play a full ninety," he added.
Onyewu, who was the lynchpin of the American defence during qualification, suffered a serious knee injury in the final qualifier against Costa Rica in October and did not play for his club AC Milan for the rest of the European club season.
The centre-half made his return to action in last month's friendly against the Czech Republic and also featured during the build-up games against Turkey and Australia but has not played a full game as coach Bob Bradley rotated his back-line.
"Obviously I would have loved to have been able to see if I could play a full ninety but as it is, a lot of the defenders didn't play a full ninety," he said.
"I think Bob has been trying to disperse the time as evenly as possible amongst the players and I don't have an issue with that," he said.
Onyewu struggled in his return match against the Czechs but has shown improvement in the subsequent games, even if he is not yet back to the dominating presence he was before the injury.
"It was a progression, the Czech game was my first game back after seven months and I could have anticipated that not being the greatest outing but you get better as you go and right now I feel fine and ready for this tournament," he said.
The only other injury worry for the U.S is striker Jozy Altidore who returned to training on Sunday after suffering an ankle injury in training last week.
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