Hans-Joachim Watzke says that the club are working round the clock to find a replacement for the Poland international following confirmation of his summer move to Bayern Munich
The Bavarians announced on Saturday that the forward will join them on a five-year deal once his contract with BVB expires this summer.
Watzke, reacting to the news of Lewandowski's departure, confirmed that he was fully expecting the announcement and explained that the hunt for a suitable substitute is on.
"This is not surprising, it is a completely normal process," the 54-year-old told Bild am Sonntag.
"In recent weeks it had been clear that it would come to this. Our sports director, Michael Zorc, is working flat-out to find an alternative."
Lewandowski, who is currently joint-top scorer in the Bundesliga with 11 goals so far this season, had long been expected to move to the Allianz Arena and Watzke stressed that he had no concerns over the 25-year-old's commitment to the club for the remainder of the campaign.
"Between us and Robert there is no problem," Watzke stated. "I have not the slightest doubt that there will be any changes to his strong performances."
Bayern sit at the top of the table with 44 points, 12 ahead of fourth-placed Dortmund.