That is the opinion of football manager Slaven Bilic.
The Croatian, who has managed West Ham United and other top teams, tried to sign Carvalho in 2017.
Leicester are now linked with a move for the defensive midfielder.
Bilic told Portuguese media outlet Record: "I saw the metronome of the team, I saw the player who is very calm on the ball, who knows all the time where he is and where the opponent is. He's always between the ball and his goal.
"He's the player that can switch sides, that is always available - cool, he doesn't lose the ball.
"He's very good in transition - the offensive one passing the ball and the defensive one stopping the opponent. He doesn't have to always necessarily take the ball but he slows the opponent down.
"He's very clever when to come between two centre-backs and protect your box when one is leaving their position.
"He's giving the team a balance. Every position in football is important but especially that one. He's keeping the shape of the team. Every team has it.
"Before, you were dividing those players. Maybe you had two midfielders and one was better defensively but that guy was not always as good on the ball - but he had the legs.
"Then the other one was good on the ball but not very responsible.
"Now you are trying to have a complete player for the position, and I saw that in him.
"I mean, not only me - the way he played in the Euros, it was was unbelievable, and the year after that, I really wanted to (sign him).
"I like those kind of players. They put a smile on my face."