Slowly but surely, Romelu Lukaku is silencing anyone who questioned his £75 million move to Manchester United this summer.

In seven competitive appearances for his new side, the Belgian has scored seven goals, six of which coming in the Premier League.

It's no exaggeration that Lukaku has taken everyone by surprise with his goalscoring exploits at United - even Jose Mourinho didn't see it coming.

Speaking ahead of this week's Champions League clash with CSKA Moscow, Mourinho explained to reporters that he didn't expect Lukaku to adapt so quickly.

"I knew he would always score goals, I knew he would always score more goals for us than for Everton or West Bromwich Albion. That’s logic," said Mourinho.

"But I was not expecting him to be match after match, putting the ball in the net."

Only once has Lukaku failed to find the back of the net this season: against Leicester City in the 2-0 win at Old Trafford.

His so-called 'weaknesses' are gradually turning into his strengths, such as his poor first touch, decision making and finishing.

Lukaku's potential is through the roof and it's scary to think that he's 24-years-old and only getting better.

So why has the Belgium international been staying behind at training recently? Well, according to The Sun, there's a very good reason.

Lukaku has been putting in extra hours to work with two of his teammates on their goalscoring: Marcus Rashford and Anthony Martial.

A source said: "He (Lukaku) has been an absolute diamond since he has arrived at the club. He has taken upon himself to work with the other young lads (Rashford and Martial).

"They are regularly staying behind for hours. The three of them are working on a number of things, runs, shooting, the cohesion and so on.

"Jose has been very impressed by the lengths they are all willing to go to. It has been a great start for Romelu at the club.

"Romelu is young himself, so there is a real camaraderie between the three of them. It's great to see."

United fans will love the fact Lukaku, Rashford and Martial are spending longer on the training ground together to work on chemistry.

Rashford, 19, and Martial, 21, are both hugely exciting talents and they now have the perfect mentor in Lukaku, even if he isn't much older than them.

The trio have scored 10 Premier League goals between them so far this season, which accounts for well over half of United's overall tally (17).