The 33-year-old limped from the Wembley Stadium pitch with an injury to his right ankle after 44 minutes of his Chelsea side's 1-0 win over Portsmouth, which gave the London club the league and cup double.
"Until I know what the MRI scan will reveal, I am naturally concerned," said Ballack.
Germany open their FIFA World Cup campaign against Australia in their first group stage game on 13 June and Ballack was due to have a scan on the ankle on Sunday.
Ballack left Wembley wearing a special protective cast on his ankle, but an x-ray at the stadium had already revealed he has not broken any bones.
A Chelsea spokesman said the medical staff do not think the injury will keep Ballack out of the FIFA World Cup as he prepares to fly to Sicily to link up with the rest of the Germany squad for their training camp.
Ballack picked up the injury after he was sent tumbling to the turf by a heavy challenge from Portsmouth's former Hertha Berlin midfielder Kevin-Prince Boateng.
Germany coach Joachim Low will be concerned about the injury to his captain who has won 98 caps and scored 42 goals.
"We know nothing is broken and we have to wait for the scan," said Low from Germany's training camp in Sicily.
"For us, the positive news is there is no break and of course we need him, he is a very important player and he has an incredible amount of experience."
Chelsea coach Carlo Ancelotti said that he was confident Ballack would be fit for the FIFA World Cup.
"We don't think it is bad injury. He took a kick on his ankle but there is no dislocation so we hope he will be ready for the World Cup," he said.