Jose Mourinho is reportedly fearful that Lukaku has gained too much muscle

It has been claimed that the striker has added half a stone since his £75m move

The weight added by the former Chelsea man is said to be all muscle

But Mourinho is worried that he has lost some speed which is harming his efforts

Manchester United boss Jose Mourinho is reportedly fearful that £75million striker Romelu Lukaku has lost some of his speed because he's added too much muscle to his frame.

The Sun claim that the Belgian striker, who joined Old Trafford in the summer, has gained around half a stone since his big-money switch.

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Club doctors insist the weight is all muscle rather than body fat and there are no lifestyle concerns around Lukaku.

Mourinho is reportedly concerned about Lukaku's recent bulking up and its effect on his speed

He is said to have gained almost half a stone in recent months as part of his gym regime

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However there is a concern that the 24-year-old has lost a yard of pace recently because of his bulked-up frame.

The former Chelsea man has struggled in front of goal recently and has scored just four times since the middle of October - a run of 19 games.

Lukaku hit the ground running in a United shirt but the goals have dried up in recent months

Lukaku has scored just four times since the middle of October for Manchester United

That follows his remarkable start in a red shirt, where he bagged at least once in nine of his first 10 games.

Lukaku's loss of form has coincided with United's title challenge faltering in the wake of their unstoppable rivals, Manchester City.

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Mourinho's men are 15 points behind the runaway leaders after 20 games of the Premier League season.

They're winless in their last three games in all competitions, including an embarrassing loss to Bristol City in the Carabo Cup.

United take on Southampton on Saturday at Old Trafford where Lukaku will be hoping to find form, before going up against his former side Everton on Monday.

Mourinho is said to be concerned about Lukaku's weight gain, which is all muscle and not fat