Fellaini signed a new two-year contract with United last week despite the criticism he has received from fans and Martinez says the 30-year-old is the type of player every coach would love to have.
Belgium take on Brazil in the World Cup quarter-finals and Fellaini is in with a chance to start after scoring a crucial goal in the 3-2, second-round victory against Japan on Monday.
"I think he is player that you have to work with to appreciate what he really brings," Martinez told Goal in an exclusive interview ahead of Friday's clash.
"I think he gets underestimated in his technical ability. I think he is a footballer that has got a level of technical control and ball control a lot higher than people think."
Fellaini moved to Old Trafford in the post-Sir Alex Ferguson summer of 2013 and has been a favourite of David Moyes, Louis van Gaal and Jose Mourinho in his time at the club.
"He was born to compete," says Martinez. "He is a winner. I think he finds football as a good opportunity to show how life should be. His interpretation of life is the way he runs the football game.
"As a team player he always sees that every action is the end of the world and that makes him a warrior and someone that every coach and manager would love to have in his team."
Fellaini made his senior Belgium debut in 2007 and has since netted 18 goals in 85 appearances.
At club level, he has now taken in over 150 outings for Manchester United and will be hoping to help them chase down more major honours after extending his stay at Old Trafford through to 2020.