Manchester United striker Romelu Lukaku can only get better under Jose Mourinho, according to Belgium boss Roberto Martinez.

The Spaniard, who managed Lukaku at Everton, has been impressed with the 24-year-old's development since his £75m move to Old Trafford this summer and has already seen a "big difference" in his game.

Lukaku has hit the ground running at United, scoring seven goals in as many games in all competitions for the 20-time champions of England but Martinez said the Belgian's form should not have come as a surprise to anyone.

"It never crossed my mind that it would be something difficult for Rom," he told talkSPORT.

"Everyone that plays with him or works with him realises very quickly that he has an elite brain.

"As a footballer, you can have your attributes but the focus he can have is quite unique. When he went to Manchester United he was solely focused on starting well and making a big impact and that is what he has done."

Despite being only 24, Lukaku has already scored 90 goals in 191 Premier League appearances but some questioned whether he would be good enough to become United's main attacking option as he was not perceived to be as clinical a finisher as some of his peers.

The Belgian's link-up play was also called into question amid suggestions Lukaku's first touch was too prone to let him down, but the step up to United has so far been seamless for the former Everton striker.

While his improvement has been evident, Martinez believes the Belgium international can get even better.

"He is still a very young player but his goalscoring record is sensational and he is a player that wants to carry on developing and carry on learning," he added.

"Now he is with a manager with whom he will keep learning different aspects at club level. I have seen a big difference in terms of his maturity when he comes to international camps.

"From the young player I signed from West Bromwich Albion and Chelsea at that time, it is the same goalscorer but with an incredible maturity. That is great to see and he still has many, many years to develop."

Lukaku was an unused substitute on Wednesday night (20 September), as United began their defence of the League Cup by thumping Burton Albion 4-1 in the third round of the competition, but he is expected to return to the starting XI when United face Southampton on Saturday.