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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Chelsea have struggled to build momentum in the opening stages of the campaign and are currently looking over their shoulders at the relegation zone.
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