The 24-year-old has just 16 months left to run on his contract at Westfalenstadion, and has long been linked with a move to the Premier League leaders after a return of 36 goals in his last 54 Bundesliga appearances helped Dortmund to the league title last term and second place this year.
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"Robert will not renew his contract, he is either [leaving] this summer or in the summer of 2014," Zorc told German broadcaster Sky.
Bundesliga rivals and runaway league leaders Bayern Munich have also been heavily linked with the striker in recent weeks, but Lewandowski's agent Cezary Kucharski confirmed to Goal.com last week that United are firmly in the race for his signature.
He said: "Many clubs follow the player but really it is only specifically Manchester United."
The Pole is thought to only command a fee of around £14 million and could signal the end of Chicharito's career at Old Trafford, with Atletico Madrid prepared to pay up to £15m for the Mexico international.