"It's a light sprain in the left foot and no one is nervous about it," a spokesman for the German Football Association said.
Ballack, who will lead out the hosts in the opening match of the World Cup on June 9, suffered the injury early in a practice match against Servette Geneva's under-18team.
Germany play Luxembourg in a friendly in Freiburg on Saturday. It was not immediately clear whether Ballack would be able to play.
The hosts take on Japan and Colombia before playing the first match of the World Cup against Costa Rica in Munich on June 9.