United fans are hoping the Portugal international can provide a creative spark in order to break down deep lying defences.
And while Fernandes has the ability to score and assist, current Rio Ave boss Carvalhal insists the 25-year-old is a box-to-box midfielder.
"He (Fernandes) is not a Number 10," Carvalhal told Sky Sports.
"He is a midfielder but he has developed that ability to break the line of the defence to score goals.
"He shoots, he assists, the passes and the free-kicks. But he is not a 10, he is an 8. He is a box-to-box player who understands very well the game. He doesn't just play. He understands everything that happens around him."
He added: "He can organise the team, he can talk, he can give feedback.
"He is kind of like a manager on the pitch. He can organise things in the last part of the pitch. He is a really exciting player.
"He has been the best player in the Portuguese league for two years minimum. He is in that group of players who are among the best in the world.
"Sometimes these technical players, it is their quality in the defensive transition that is a problem in the Premier League. When they lose the ball, they don't react quickly enough to win it back, but he has developed a lot in the past two seasons.
"When his team loses the ball, he reacts quickly to win the ball back. That is important in England. He is not lazy, he is the opposite. A fighter. He has everything to be a key player."