Benfica recorded a 3-0 victory over the Catalan giants in Portugal, with Darwin Nunez registering twice and Rafa Silva also getting his name on the scoresheet during a strong performance.
According to A Bola, officials from the 20-time English champions were in Lisbon with the view to scouting the home side, who have also made a strong start to their domestic campaign, winning all seven of their league games.
The report does not mention any particular individuals that were being watched by Man United, but Nunez hit a brace in an impressive performance, while Julian Weigl was among those to impress.
The Red Devils were also victorious in the Champions League on Wednesday night, with Cristiano Ronaldo's late goal seeing them record a 2-1 win over Villarreal.
Ole Gunnar Solskjaer's side will now be looking to return to winning ways in the Premier League when they welcome Everton to Old Trafford on Saturday afternoon.
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