The 30-year-old has consistently denied reports he is set to leave the reigning Barclays Premier League champions this month, but was spotted on Sunday boarding a flight to Frankfurt, where he is expected to hold discussions with his former club.
Red Devils boss Sir Alex Ferguson has an array of attacking options to pick from at Old Trafford, with Wayne Rooney and Danny Welbeck currently the first-choice picks ahead of Javier Hernandez and then Berbatov.
Having fallen down the Manchester United pecking order, the former Bulgaria international has attracted interest from a host of clubs across Europe, and after finishing the 2010-11 campaign as the league's joint-top goalscorer, there is no shortage of interest in Berbatov's signature.
Berbatov left Leverkusen in 2006 to join Tottenham, before moving on to Manchester Untied in 2008 in a £30.75million switch, but if sold he is unlikely to command anywhere near that figure.
However, the Bundesliga outfit's hopes of re-signing their former star striker after it emerged that German rivals Bayern Munich want to sign Berbatov as a replacement for Croatian forward Ivica Olic, who is expected to leave the Allianz Arena in the not too distant future.