Manchester United, Arsenal and Everton were among a host of clubs scouting at Benfica's 2-1 derby win over Sporting Lisbon on Sunday, sources close to the Eagles have told ESPN FC.
Scouts from United have been regular visitors to Portugal this season as they keep a close eye on Benfica trio Victor Lindelof, Nelson Semedo and Goncalo Guedes.
Red Devils boss Jose Mourinho is actively looking to bolster his defensive options in January and Swedish centre-back Lindelof is reportedly among his top targets.
Right-back Semedo is also on United's radar as Mourinho looks to bring in a new full-back with Antonio Valencia filling in this season.