Manchester United scouts were present in Portugal at the weekend to watch Benfica trio Nemanja Matic, Ezequiel Garay and Nicolas Gaitan play in the 1-0 victory over Sporting Braga, reports A Bola.
According to Sport Witness, representatives from the Premier League champions have been frequent visitors to the capital this season, as they also continue to monitor the progress of William Carvalho at Sporting Lisbon.
Red Devils boss David Moyes is ready to ramp up his transfer interest, with Matic pinpointed as the club's primary target for a January switch to Old Trafford.
The 25-year-old midfielder has long been linked with a move away Estádio da Luz, with former club Chelsea and Real Madrid also reportedly monitoring his situation with the Primeira Liga giants.
Matic's future could be determined by Benfica's success in the Champions League this season, with Jorge Jesus's side currently residing in third place in Group C, behind Paris Saint-Germain and Olympiakos.
Failure to progress to the knockout stages could force the club to sell the Serbia international in order to balance their books, and Manchester United will hope to be in pole position for his signature.