Manchester United manager David Moyes reportedly sent scouts to watch Porto defender Eliaquim Mangala in action last weekend.
Mangala has attracted interest from a whole host of European clubs in the last 12 months, with Chelsea among the sides believed to be interested in the French youth international.
United were at Porto's match against Maritimo on Sunday to watch Mangala start in central defence, as the Portuguese champions ran out 3-0 winners.
Barcelona also had scouts at the match but both sides face competition from Chelsea for Mangala's signature, after the Blues had a bid rejected by Porto earlier this summer, according to The Mirror.
The 22-year-old has become a regular starter for Porto since signing from Standard Liege in 2011, scoring eight goals in 80 appearances.
