The Blues have been heavily linked with a move for the Serbian international, who has gone on to establish himself as one of the most promising midfielders in Europe.
Matic moved to Estadio da Luz as part of the £25million deal which brought Brazilian defender David Luiz to Stamford Bridge in January 2011, under the former management of Carlo Ancelotti.
Now Jose Mourinho wants to bring the 25-year-old back to west London, in a bid to strengthen his options in central midfield.
Red Devils boss David Moyes has also been credited - by the London Metro - with an interest in bringing Matic to Old Trafford, but it's understood that the former Chelsea star would favour a return to his old club, having already told Benfica he wants to move back to the Premier League.
It's not yet clear whether the Primeira Liga giants will accept Chelsea's £21million offer, with the Portuguese club likely to hold out for a fee closer to his reported £37million buyout clause.