Schalke general manager Horst Heldt insists Peru international Jefferson Farfan is not for sale.
The former PSV Eindhoven forward is reportedly a target of Juventus, who are continuing to remodel their squad.
Speculation has increased given that the 26-year-old has less than 12 months remaining on his contract, but Heldt is eager to agree a new deal.
He told Reviersport newspaper: "Jefferson Farfan will definitely not be leaving us within this current transfer window. That is for sure.
"In contrast, we are still working very hard to ensure we are able to sign new terms with Jefferson to keep him at our club for much longer."
Farfan joined Schalke in the summer of 2008 for approximately £8million from PSV.