Heldt has also revealed that the reason the club refused to allow the midfielder to leave was because there was no replacement in the market.
Speaking to Sky Germany, Heldt said: "There were offers for Julian this winter.
"We told him, but also that we wouldn't let him go. There was no replacement in the market and Julian didn't really want to leave. But he would have brought us a lot of money.
"The moment will come. We can't avoid this. He's an outstanding player in Europe - one day he'll leave us."