United have agreed to pay Everton £75 million for the striker, whose transfer from Goodison Park to Old Trafford was confirmed on Monday.

But a series of add-on payments built in to the fee could take Lukaku's price tag above the £89.3 million United paid Juventus for Paul Pogba last summer.

Everton are due two payments of £7.5 million each if certain criteria are met during the span of Lukaku's initial five-year contract, which could increase the cost of the deal to £90 million.

Sources close to Everton have suggested the targets that would trigger the add-ons are "achievable."

United have negotiated deals in the past which include bonus payments for goals scored, international appearances and winning major trophies.

In April, Anthony Martial triggered an £8.4m payment to his former club Monaco when he scored his 25th United goal. The French side will be due further payments if Martial makes 25 appearances for France or his nominated for the Ballon d'Or.

Lukaku trained with his new teammates for the first time on Monday as United began their preseason preparations at UCLA.

The announcement that the 24-year-old's deal had gone through came out just an hour before he was due to start training.

"Romelu is a natural fit for Manchester United," said manager Jose Mourinho. "He is a big personality and a big player.

"It is only natural that he wants to develop his career at the biggest club. He will be a great addition to the group. I am really looking forward to working with him again."

Lukaku added: "When Manchester United and Jose Mourinho come knocking at the door it is an opportunity of a lifetime and one that I could not turn down.

"You could see the fight, determination and the spirit in this team during the Europa League final and I want to become a part of that."