The 24-year-old, who has scored 33 goals in 39 appearances this season, has a year remaining on his contract with the German club and has been strongly linked with a move to Old Trafford at the end of the campaign.
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"I can tell you only one thing - Robert is interested in the future coming to the Premier League," Barthel told the Guardian.
"The Premier League is very interesting for Robert. How long is the future, I cannot say to you. I think this summer is not the future."
Goal.com revealed last Tuesday that Bayern Munich have edged ahead of United in the race to sign Lewandowski, as Juventus are lining up a deal for Mario Gomez, which will create space in the Bundesliga champions' squad for the Pole.
When asked whether Lewandowski will join Bayern Munich, Barthel replied: "I don't know."
On Sunday, Dortmund general manager Hans-Joachim Watzke conceded the club will discuss "decent offers" for Lewandowski but claimed the decision does not mean the forward will leave Jurgen Klopp's side.