The Red Devils were keen on a move for the 24-year-old in the summer, however their interest cooled once Robin van Persie arrived from Arsenal.
Lewandowski's current contract expires in 2014 and talks over a new deal appear to have stalled.
"We will sit down and discuss what to do next if Robert does not extend his contact," said Dortmund chief executive Aki Watzke.
According to The Mirror, United boss Sir Alex Ferguson remains a long-term admirer of Lewandowski and has asked to be kept informed of the player's situation.
Lewandowski has been capped 49 times by Poland at senior international level, scoring 15 goals.