The 26-year-old joined on a season-long loan during the summer, and it has recently been reported that the winger was interested in completing a permanent move to the Hornets.
However, Carrillo has insisted that he only left Benfica to secure first-team football, and will return to the Portuguese giants at the end of the 2017-18 campaign.
"I had to go out for more minutes, but I have the best memories of Benfica, to which I will return next season with more experience," Carrillo told reporters.
"I hope to have a good season at Watford and demonstrate my potential to merit a new opportunity at Benfica, the club that bet on me after leaving Sporting. I am very grateful for that."
Carrillo has made five Premier League appearances for Watford thus far this season.