On Wednesday, Sport Bild reported that United had bid on Hummels.
When asked if United really had made an offer for the 26-year-old, Zorc refused to comment.
"I can't confirm that," Zorc told Bild.
Hummels meanwhile made it clear that he was starting to get sick and tired of having to constantly speak about the rumours of him leaving Dortmund.
"I don't want to comment on it all the time," he said. "I've done that a few weeks ago, and that has to be enough."
However, Dortmund CEO Hans-Joachim Watzke revealed that Hummels is set to play a big role when new head coach Thomas Tuchel takes over at the end of the current season.
"The decision where Hummels will play in the new season will be made by Borussia Dortmund," Watzke said.