Belgium head coach Roberto Martinez says he left out Marouane Fellaini out of the squad for the friendlies against Mexico and Japan as the Manchester United midfielder was not yet fully fit.

Fellaini, 29, returned from a one-month knee injury layoff to play as a substitute in United's 1-0 defeat at Chelsea on 5 November – just 24 hours after he returned to full training.

Martinez said the former Everton midfielder needed more time time to build up his match fitness and would be allowed to stay in Manchester during the international break to focus on his recovery.

"He needs a proper 10 days work with no game in order for him to be fully fit," the Belgium boss was quoted as saying by Sky Sports.

"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."

Fellaini joined United from Everton for £27.5m ($36m) in 2013 and has established himself as a key player at Old Trafford under Jose Mourinho.

He started 18 Premier League games for United last season, scoring one goal, but is in the final year of his Red Devils contract.

Marouane Fellaini
Marouane Fellaini has established himself as a key player at Manchester United Getty Images

Martinez also backed Romelu Lukaku to rediscover his scoring touch at United amid a scoreless run in club football that extends back to September.

"I know Romelu inside out. I think he just relishes the challenge," he said. "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."