Manchester United were reportedly one of many clubs to have scouts in attendance for Porto's Primeira Liga fixture with Sporting Lisbon on Friday night.
The two clubs are currently competing for the Portuguese top-flight title, Porto currently holding a six-point advantage over the reigning champions.
Friday's contest at Estadio do Dragao ended in a 2-2 draw, and some of the attacking play on show is likely to have impressed interested observers.
According to O Jogo, United attended the game as they contemplate whether either club have any players who could improve the first-team squad in the short and long term.
The Premier League giants are no strangers to conducting deals with Sporting having signed Cristiano Ronaldo and Bruno Fernandes from Leoes.
The report claims that Sevilla, Atletico Madrid, Borussia Dortmund, RB Leipzig and Monaco also made the trip to assess potential transfer targets.
