Ballack was released by Chelsea earlier this week as his contract at Stamford Bridge was set to expire.
“I don’t want to make it a secret – when a player like Michael Ballack with his history with us, is on the market, then we think about him,” said Voller.
“Michael had a great time here and we have remained in contact over the years.
“We are relaxed about it all, but Michael Ballack is an issue we are dealing with, particularly since he would like to return to Germany.”
