According to talkSPORT, the scouts from both sides were in attendance during the Portuguese champions' 3-0 win over Maritimo last weekend. The Premier League champions were keen on signing Benfica's Ezequiel Garay earlier in the summer, but the deal did not take any shape.
David Moyes is yet to make a star signing this summer and signing a central defender as a future replacement for Rio Ferdinand and Nemanja Vidic is also an option, apart from strengthening the midfield.
Barcelona are in need of a centre-back, especially after ageing skipper Carles Puyol's battle with injuries. The Catalan club have been linked with moves for Chelsea's David Luiz, Paris Saint-Germain's Thiago Silva and Liverpool's Daniel Agger.
However, the La Liga giants have failed in their attempts to sign a central defender, leaving Gerardo Martino with Gerard Pique, Javier Mascherano and Marc Bartra as first-choice defenders. Barcelona are likely to give United some competition in signing Mangala, who has been one of the standout performers for Porto last season.
Any move from United or Barcelona for the French international's services could cost in excess of £20m.
Meanwhile, United winger Ashley Young is delighted to have made his return from injury during his side's goalless draw against Chelsea on Monday. The England international returned to action after missing the latter part of last season and the entire pre-season tour.
The former Aston Villa man came on as a substitute and is rearing to go under the Red Devils' new manager for a new era at Old Trafford.
"I feel ready. I didn't have a full pre-season but I have been trying to get myself fit all summer and when I'm fully fit, when called upon, I will go out there and give 110 per cent," Young told the club's official website.
"I had a stop-start season last year with the injuries I had, which was frustrating. But I was glad to get back out on the pitch and to get some minutes. I felt good out there [against Chelsea] and, fingers crossed, now I won't have the same injury problems this season," he added.
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