The 32-year-old former Serbia international has a contract with the Red Devils until the summer of 2023.
Matic previously spent two-and-a-half years with Benfica, winning one Portuguese league title, before joining Chelsea in January 2014.
Matic has made 26 appearances in all competitions for Ole Gunnar Solskjaer’s side this season and insists he is happy at Old Trafford but will not rule out a return to the Estadio da Luz if the opportunity presents itself.
In an interview with Sport TV, the 32-year-old said: "I still don't think about finishing my career. Let's see.
"For me, Benfica was always in the heart. Let's see what the future will bring.
"If Benfica ever thinks it needs me again, let's think about it, but now I'm very happy at Man United, I don't think about leaving, but in football you never know.
"If one day I return to Portugal, the only team I wanted to play in Portugal was Benfica.
"I was very happy there. Benfica is one of the best clubs in the world.
"I was very happy there in the team, with the fans and with everyone in the club.
"Whenever I have time, I like to go through Lisbon. My family also likes it a lot and misses everyone from Lisbon."
