Romelu Lukaku has reportedly been nicknamed "Rocky" by his United team-mates, due to his incredible work rate and dedication to training.
Lukaku is now struggling with an ankle injury which he picked up in the 2-1 victory over Arsenal.
"He is trying to kill two birds with one stone".
Analysing the five-times champions chances of winning the World Cup this summer, Mourinho told ESPN Brazil: 'I like very much the basic structure of Brasil, its tactical and mentality.More news: 53Lebanon Preliminary Election Results Show Victory of Hezbollah, Allies
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'They say he is more ripped than normal at present and knows he isn't going to have a full pre-season this summer. The 20-year-old is still developing and he is in the right place to see through that development. Jose is obviously loving it, as it is setting a firm message out to the others. "I would welcome him at Arsenal".
Lukaku started his journey with Chelsea in 2009 but he could not make an impact as he was loaned to West Brom after failing to score in 10 senior appearances.
Martial joined United from Monaco in the summer of 2015 and scored 17 goals in his debut season, but he has struggled to maintain that early form, with just 8 goals last season and 11 in this campaign. No one can deny the Frenchman is one of the most talented youngsters in the world, but he hasn't been able to secure a starting role yet.