Michael Zorc has revealed Borussia Dortmund cannot afford to buy a like-for-like replacement for Robert Lewandowski.
The club confirmed last month that the striker had chosen not to extend his stay at Signal Iduna Park beyond 2014.
Bundesliga rivals Bayern Munich and Premier League leaders Manchester United have both been linked with a summer move for the striker, but the sporting director says the idea of bringing in a player of similar ability is not realistic.
"Economically, that's not imaginable for Borussia Dortmund," the 50-year-old 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."
But with the club preparing for the second leg of their last 16 Champions League clash with Shakhtar Donetsk, after drawing the first leg 2-2 in Ukraine, Zorc believes advancing to the quarter-finals would be a big boost to the club.
He added: "Our goal is to reach the next round. We have built us a solid foundation with the 2-2 three weeks ago.
"Should we reach the quarter-finals, which we expect from us, it would be an important step in establishing ourselves internationally."