Wayne Rooney has pinpointed mental strength as the main test for Romelu Lukaku at Manchester United and warned that it is harder for today's new signings at Old Trafford than when he joined the club in 2004.

Rooney, who last week rejoined Everton as Lukaku moved in the opposite direction, told the 24-year-old Belgian that any weakness will be exposed at England's biggest club.

'Playing for Manchester United is a test of character,' said Rooney. 'You've got to be strong enough mentally to accept the challenge. United are a football club that demands success. Romelu has to be strong enough to take up that challenge.

Wayne Rooney says Romelu Lukaku will have to show no weaknesses at Manchester United

Rooney has warned the Belgium international to expect a 'test of character' at Old Trafford

'If he does that then, with the ability he has got, then he will be a success. If he worries about things then it will be more difficult.'

Rooney was just 18 when he signed for United and went on to become the club's leading goalscorer, winning every English and European trophy available.

But he acknowledges the help given to him by a dressing room dominated by the 'Class of 92' is something Lukaku won't have, after the club's period of transition with four managers in four years.

'There are standards that you have to keep when you are at Manchester United. That came from Sir Alex when I went to United and it was passed down through the likes of Giggsy, Gary Neville and Scholesy,' said Rooney.

United manager Jose Mourinho keeps a watchful eye over Lukaku during training in America

Paul Pogba played a role in persuading Lukaku to choose United over a return to Chelsea

'That has been more difficult over the last few years, with some of the players who joined the club. That's not my problem anymore. I'm just ecstatic to be back at Everton.'

Rooney made a goalscoring return for Everton in Tanzania on Thursday and manager Ronald Koeman wants the 31-year-old to exert his leadership qualities.

'I need to get in the dressing room and learn about the players here,' said Rooney. 'We've got a good group. I've played with a few lads at club level and international level and there is also some young lads. It's a good mix.'

On Ross Barkley, whose future at Goodison Park is uncertain, Rooney added: 'With the ability Ross has got, of course we all want him to be a part of what we're trying to achieve at Everton.

'He can help us get better. But the decision about his future is down to him.'