Guimaraes were held to a 1-1 draw with Belenenses as Manchester United watched Bernard Mensah.
According to Skysports reports, Mensah comes highly recommended by super-agent Jorge Mendes, who represents United boss Jose Mourinho and Old Trafford legend Cristiano Ronaldo.
Mensah was linked with a move to United in the summer of 2015 before being snapped up by Atletico Madrid.
Scouts from United, AS Monaco and Sevilla were also in attendance.
The Ghanaian started off at the Portuguese club before catching Atletico manager Diego Simeone‘s eye last summer.
Since his La Liga move he has been shipped out on loan to Getafe and is back to Portugal on a season-long loan deal with his former side Vitória Guimaraes.