Chelsea striker Romelu Lukaku is so powerful that he is unable to last a full 90 minutes yet, according to his Roberto Martinez.

Lukaku has been impressing since going on a season-long loan at Everton from Stamford Bridge on transfer deadline day, but the Toffees boss says he still has work to do on the physical side of the game.

Chelsea signed the Belgium international for around £20million from Anderlecht in 2011 and, such was his size at 18 years old, his physicality was not one of the concerns many people would have had about him.

However, Martinez has revealed that the while power in Lukaku's game is one of his biggest assets, it also means he cannot yet effectively maintain his level for the entirety of a match.

“Romelu is a young man playing with incredible strength and power, so it takes a little bit of time adapting your body to be able to cope for 90 minutes,” he told reporters.

"That is something that we are working with him on. Against Newcastle for the last 15 minutes he probably should have come off, but he had to stay on the pitch so that mentally he could overcome that target."

Everton have started the Premier League season in good form, losing only a single game and Lukaku has already found the net four times in four appearances.

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