The 25-year-old spent two years at Chelsea before joining Benfica, but did not start a game for the Blues.
Matic has become a key player at Benfica since joining in the summer of 2011.
The Mirror reports that Matic has been watched by United scouts in his last two games.
United could make a move for Matic in January, but face competition from Italian giants Napoli.
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The start of the 2017-18 Premier League season is just around the corner and is shaping up to be one of the most competitive yet.
As always it is the so called “big clubs” that have splashed the most cash in the transfer window so far this summer. Manchester United or City? Arsenal or Tottenham? Who are the winners and losers of the transfer window so far?