The report claims United were represented at the Coimbra de Mota Stadium, along with representatives for Everton, Celtic, Kaiserslautern, Cologne, Chievo and Panathinaikos.
Benfica and Sporting scouts were also in attendance, although the report fails to say who the focus of all the attention was on.
Former-PSG star Kevin Gameiro climbed off the bench to help Sevilla record a 2-1 win over their Portuguese opponents in the Group H encounter.
Vitolo opened the scoring for the La Liga side, only to then see Estoril immediately peg them back through Bruno Miguel.
But Unai Emery's side secured a precious three points through France international Gameiro, whose low shot beat Brazilian stopper Vagner into the bottom corner.
Sevilla beat Manchester United 3-1 in preseason - a match that also doubled as Rio Ferdinand's testimonial at Old Trafford.