The Belgian has made a big impression since joining the Toffees on a season-long loan from Chelsea in August and has already scored five goals for the club.
Martinez feels the player's fitness is continually improving, as he showed by playing the full 90 minutes of last weekend's win at Aston Villa.
Martinez told the club's website: "I do feel he is getting stronger for longer in games and, remember, Rom plays with real power and that's the way he should be playing every time he's on the pitch.
"It was positive for him to finish the whole game against Villa.
"Now he must try to carry on working hard and make sure that his strength and power can be there to help the team for the full duration of the game."
- Sports & Recreation