Manchester United had scouts posted for Benfica's reserve-team game with Academico de Viseu.
A Bola says representatives from United, Atletico Madrid and Genoa were all present for Sunday's game.
The subject of their attention was Benfica defender Branimir Kalaica, 18.
The Croat, who plays as a fullback, was snapped up by Benfica last summer from Dinamo Zagreb, and is an U19 international.
Standing at 192 cm, Kalaica can also play at centre-half and in midfield.