The midfielder will undergo tests Sunday to determine whether he will be fit for Germany's World Cup opener on June 13 against Australia.
"It's not that good at the moment, but I have to wait until tomorrow for the scan, and then we will know more," Ballack said. "I went off and couldn't carry on, but it's not that bad."
Ballack went down clutching his ankle after a challenge from Portsmouth's Kevin-Prince Boateng. Ballack hobbled off in the 44th minute.
"It's not good in football when that happens, and nobody wants to see it," Ballack said. "He got away with a yellow and was lucky."
"We don't think it is a bad injury," Chelsea manager Carlo Ancelotti said. "We hope he will be ready for the World Cup."