According to Portuguese newspaper Record, the Premier League leaders had scouts present at Estadio da Luz for the club's top-of-the-table Primeira Liga clash with FC Porto last night, a match that finished 2-2, and left the title race wide open.
Red Devils boss Sir Alex Ferguson is a long-term admirer of Argentine international Gaitan, with the 24-year-old heavily linked with a £30million move to Old Trafford during the summer.
Despite the proposed transfer falling through, Manchester United are still in the market for a new wide-man, as speculation continues to surround the future of Portuguese winger, Nani, and are ready to reignite interest in the former Boca Juniors star.
While in Portugal, Ferguson's scouts were also reportedly told to assess the ability of Garay - a former Real Madrid player who has rediscovered his best form in Lisbon.
The 26-year-old could also prove to be an expensive purchase, though, with Benfica obliged to hand over half of any transfer fee to the La Liga giants.
Manchester United are not renowned for being particularly active during the winter window, so if their interest in Gaitan and Garay comes to nothing this month, there is a chance it could be followed up at the end of the campaign.