This is according to ESPN FC, who claim the Red Devils scouted the defender in Benfica's clash with Estoril on Wednesday.
The report adds Arsenal and West Brom representatives were also in attendance.
Manchester United are still in pole position to sign Benfica's Victor Lindelof
Lindelof played the full 90 minutes for Benfica, as his side were held to a 3-3 draw.
Manchester United are long-term admirers of Lindelof.
The 22-year-old was heavily linked with a £38million move to Old Trafford back in January.
Manchester United wages: Which Red Devils star earns the most?
Thu, April 6, 2017
Express Sport runs through how much each Manchester United player earns per week, according to Total Sportek/Sky Sports News
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How much do Jose Mourinho's Manchester United players earn each week?
But the right-back decided to stay in Portugal.
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And he extended his contract with Benfica until the end of the 2020/21 season.
But Jose Mourinho remains interested in the defender and will now reportedly launch another move this summer.