Martinez first worked with Lukaku at Everton when the striker was just 19 years old and is now his national team boss, with Lukaku having since moved on to Manchester United at club level.
The 25-year-old has impressed at this summer's World Cup with Belgium reaching the semi-finals, and ahead of Saturday's third-place playoff against England, Martinez hailed the Golden Boot contender as the very best in his position.
"If you ask me 'is he the best in the world?', yes," Martinez told reporters at his pre-match press conference.
"But I am biased. I worked first with him when he was 19 and I've seen an incredible, incredible journey in his career.
"His numbers throughout his career are very difficult [to achieve]. His biggest attribute is he is a goalscorer and a finisher."
Lukaku has failed to score in his last three games for Belgium, but has 23 goals from his last 23 international appearances.