Fernandez finished the season as a first-choice for manager Enzo Maresca and a Europa Conference League winner.
Poyet said of the Argentina international to Tribalfootball.com: "Well, I think it went from being okay to being very, very important. I think that now everybody can see probably the position. You know, when Fernandes arrived, everybody wanted him to play next to (Moises) Caicedo and be a double six and being very, you know, sitting in front of the back four.
"And maybe he needed to be alone in that position, where he was not because Caicedo was next, or being a little bit more free. And I think, look, I'm not going to compare it with me because he's more defensive than me, or I'm more offensive than him.
"But I think that a few months in that more offensive position and a little bit more freedom, and because it looks to me that he's a very intelligent player, he started using that as a plus for his game.
"He took advantage of that position and that freedom to use it in a better way and he's been very influential in the last three or six months, which is important. And also started collaborating with some goals. So he's becoming a player that everybody now is looking to play with and to be worried when you play against."
- Gus Poyet was speaking to Tribalfootball.com on behalf of BetBrain