Manchester United manager David Moyes sent a representative to scout a number of Porto's first team stars on Sunday, according to Sport Witness.
Sport Witness claim Manchester United, Marseille and Sassuolo were all at the Estádio do Dragão to cast their eye over the likes of Alex Sandro and Eliaquim Mangala.
Manchester United have been keeping a close eye on French centre-back Mangala for a while now and some reports even suggested that the Premier League champions have contacted Portuguese super agent Jorge Mendes to try and finalise a deal for the 22-year-old.
Sport Witness also claim Manchester United are interested in signing Brazilian midfielder Fernando, who is out of contract at the end of the season, Lucho Gonzalez and highly sought-after full-back Sandro.
Interest in Porto's aforementioned defenders may have intensified over the weekend - following Manchester United's 3-1 defeat to Chelsea.
However Moyes may choose to wait until the summer to sign both Mangala and Sandro as current defenders Nemanja Vidic and Patrice Evra could both leave the club at the end of the season.
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