The report claims United and Liverpool were two among several clubs that sent representatives to the game in Portugal, reportedly to watch Argentine duo Ezequiel Garay and Nico Gaitan.
Tottenham - Benfica's next opponent in the Europa League - also had scouts in attendance, as did Everton, Manchester City, Celtic, Brighton and West Bromwich Albion.
According to the report, Milan, Atalanta, Sevilla, Montpellier, Borussia Dortmund, Stuttgart and Elche also had officials in the stands.
Garay and Gaitan have long been linked with United, while Liverpool were also thought to be running the rule over transfer targets ahead of the summer transfer window.
Brendan Rodgers was foiled in his bid to Dnipro winger Yevhen Konoplyanka in January, but the Anfield boss is expected to be busy over the summer - especially if Liverpool manage to clinch a Champions League qualification spot.
Likewise the United board is ready to back David Moyes with a sizeable transfer kitty as the Old Trafford manager begins the process of reforming the squad inherited from Sir Alex Ferguson.