Dortmund revealed on Thursday that the Poland international, who has been linked with Bayern Munich and Manchester United recently, will not be signing a new deal with the club in the summer.
Zorc has said that bringing in a player of similar ability to Lewandowski is unrealistic.
"Economically, that's not imaginable for Borussia Dortmund," he told Kicker.
"We would need to sign an absolute world-class player to match the quality Robert Lewandowski embodied in the three years he's been playing for us."
Lewandowski has hit 21 goals in 32 appearances for Dortmund this season.