Juve head coach Massimiliano Allegri has hinted Pjaca would be allowed to leave on a temporary basis, having failed to make a single Serie A appearance this season.
Schalke appear to have won the race to sign the Croatia international, with the completion of a loan move expected imminently.
"As things stand, Marko Pjaca will be joining us tomorrow [Thursday]," Heidel said.
"Everything has been agreed verbally, and we have the medical lined up for tomorrow."
Pjaca returned from eight months out due to a cruciate ligament injury with the Bianconeri's youth team in November, but has been unable to break into Allegri's plans.
The 20-year-old forward signed a three-and-a-half-year contract with Schalke on Wednesday.
Schalke return from the Bundesliga's mid-season break with a trip to RB Leipzig on January 13.