The Spain U21 star has been linked with a return to Old Trafford, with his deal carrying a buy-back option for United.
But Carreras told Marca: “The truth is that, with confidence, a player changes a lot. Since I arrived, I have played everything and I am on top. The important thing is to continue to win games, but individually I feel very good. Now we go much more to the goal, more to the goal, which follows the shield of Benfica and for a few years we have not seen so much. Now we are very well and aiming for everything.
“I am focused on what I have to do, because every three days I have a game. It's always beautiful to hear from teams and everyone wants to grow, but I'm very well here.
"United have a purchase option that was signed, but I'm very happy here. I have five years of contract with Benfica and I want to win titles and continue to grow personally, I am still 21 years old. I want to win, either with Spain or Benfica, to continue to grow.”