Wayne Rooney has said Romelu Lukaku must be mentally strong to succeed at Manchester United and warned it is increasingly difficult to live up to the high standards set at the club during Sir Alex Ferguson's reign.
Rooney, United's all-time record goal scorer, believes Lukaku has the ability to become a success at Old Trafford but said that is not the deciding factor.
"Playing for Manchester United is a test of character," Rooney told reporters. "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.
"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.
"Hopefully, Romelu has got people there who will give him the right guidance and get the best out of him."
Rooney added that the amount of change seen at United in recent seasons means maintaining high standards has been difficult as the club looked to rekindle the success seen during Ferguson's time in charge.
"There are standards that you have to keep when you are at Manchester United," he said. "That came from Sir Alex when I went to United and it was passed down through the likes of Giggsy [Ryan Giggs], Gary Neville and Scholesy [Paul Scholes].
"Over the last few years, it was down to me and Michael Carrick to keep the new players maintaining those standards.
"That has been more difficult over the last few years, with some of the players who joined the club. There are traditions at United that have to be maintained.
"It has become harder, but that's not my problem any more. I'm just ecstatic to be back at Everton."
Rooney is keen for Ross Barkley to remain at Everton amid speculation he could depart this summer.
Everton manager Ronald Koeman had said the midfielder, who has one year left on his current deal, must sign a new contract or leave.
"With the ability that Ross has, of course we all want him to be a part of what we're trying to achieve at Everton," Rooney said.
"He can help us get better but the decision about his future is down to him. I'm sure he will speak to the manager and do what's right for him."