According to the Daily Mirror, the defending Premier League champions have stepped up their interest in the midfielder after watching him closely in his last two games.
The 25-year-old spent three seasons at Stamford Bridge between 2009 and 2011 but left Chelsea for Benfica as part of the deal that brought David Luiz to west London.
But he's become one of the most sought-after midfielders in Europe after impressing for the Portuguese giants domestically and in the club's run to the Europa League final last season.
Manchester United scouts have been paying close attention early this term, and the report claims they're ready to make their move for the Serbian international.
He's reportedly got a €50m release clause in his contract but Benfica would be willing to accept £20m.
United boss David Moyes is thought to be keen to strengthen his midfield in the January transfer window despite the addition of Marouane Fellaini over the summer.
But Napoli are also interested in the ex-Chelsea star and could rival United for his signature.