Lukaku told critics to judge him when he is the "finished article" after failing to score in seven matches across all competitions, following a stellar start to life at Old Trafford when he found the net 11 times in his first 11 games.

The £75m summer signing has received the backing of his former Everton and current Belgium boss Martinez, though, who insisted he is unconcerned by Lukaku's loss of form and defended the striker's resolute mentality.

"I've got a big advantage with Romelu because I've worked with him for four years," said Martinez ahead of Belgium's World Cup warm-up matches against Mexico and Japan.

"I know Romelu inside out. I think he just relishes the challenge. He's been enjoying a really good start to the season and now is a period he is ready to cope with.

"I have no worries whatsoever. All he needs to do in his career is to carry on being himself in a club with the expectations and the demand of Manchester United.

"For me nothing has changed in Romelu's mind. He is confident, he has an incredible belief in what he can bring.

"All I want is that he carries on seeing himself in that manner when he scores and when he doesn't, which is going to be highlighted in a club like Manchester United."

Lukaku's team-mate Marouane Fellaini has been left out of Belgium's squad due to a "medical decision", according to Martinez, despite the midfielder's appearance in United's 1-0 defeat to Chelsea on Sunday.

Marouane Fellaini needs 10 days without a game to reach full fitness, says Martinez

Fellaini has been allowed to focus on returning to full fitness back in Manchester ahead of United's game against Newcastle on November 18.

"He needs a proper 10 days work with no game in order for him to be fully fit," said Martinez.

"Medically it is the right choice to allow Marouane to get fit. If it had been a competitive game I think we would have called him up because maybe for the second game he could have been on the pitch.

"This is a period where we can afford not to risk him until he is 100 per cent."