The 24-year-old Polish international was heavily linked with a move to the Premier League giants during the summer, with an offer of £16million believed to be on the table.
But, after failing to negotiate that price with the reigning Bundesliga champions, Red Devils boss Sir Alex Ferguson opted instead to sign Robin van Persie in a £24million deal.
Lewandowski is under contract in Germany until 2014, but Dortmund officials were keen to open negotiations with their star striker, in a bid to end speculation linking him with Old Trafford.
However, after talks between the two parties seemingly broke down, there is once again a wave of uncertainty surrounding the player's future, with reports suggesting the club could be ready to cash-in.
"We will sit down and discuss what to do next if Robert does not extend his contact," confirmed Dortmund chief executive Aki Watzke.
Manchester United now boast one of the most fearsome attacks in the Premier League, with Wayne Rooney, Danny Welbeck, Javier Hernandez and Van Persie giving Ferguson a selection headache almost every week.
It has been questioned whether or not the club require another option upfront, so Lewandowski's prospective arrival could prompt the departure of one of United's existing stars.