Can is coming towards the end of his contract at Liverpool, with his current deal expiring in summer 2018.
Goretzka has netted four goals in nine appearances for Schalke this time out.
In the summer of 2013 he was snapped up by Schalke - and the rest is history.
Speaking at a RevierSport event, Heidel said: "I trust Leon when he says that he has not yet taken a final decision".
"We have no deep problems, but we have to look ahead, knowing that we need to be competitive and seize any opportunities that the market offers us", chief director Beppe Marotta told Sky Italia.
But it's his relationship with his boss at club level that Heidel thinks may tempt the midfielder to stick in Gelsenkirchen, despite interest from a number of Europe's leading clubs.
Schalke have seen several talented youngsters leave in recent seasons, including Julian Draxler, Leroy Sane and Joel Matip, who left on a free transfer in 2016.