The Belgian striker is expected to be unveiled as a Red Devil on Monday after a £75million transfer deal with Everton was agreed.
Premier League champions Chelsea had been showing substantial interest in Lukaku, who left Stamford Bridge in 2014, but United secured his signature after a dramatic move.
Speaking about the deal for the first time, Lukaku told ESPN: ‘This is an opportunity that I’ve always wanted since I was a child.
‘When [Manchester United] came calling, I didn’t have to think twice. So I’m really delighted to hopefully be part of their history.’
When asked why he chose United over the possibility of a return to Chelsea, Lukaku said: ‘Who would say no to the biggest club in the world?
‘Best stadium in England. The best fans. This is the perfect opportunity. I always said I wanted to play for a team that’s challenging for every trophy that there is.’
He added: ‘I think Manchester United at the minute wants to be the dominant team, that dominant force.
‘If you look at their history, it says enough. To become part of a club like them is something that I really wanted. I’m thankful for the opportunity that they gave me.
Romelu Lukaku's Premier League season Games played37Goals scored25Assists6
‘Now it’s time to work hard, work harder than I ever did before. I’m willing to do it. Hopefully, it will come a good way.’
Lukaku will soon link up with Mourinho, the manager who sanctioned his departure from Stamford Bridge three years ago after a frustrating spell at Chelsea.
But the 24-year-old insists there is no bad blood between the pair and says he is thankful for Mourinho for giving him the opportunity to join United.
‘[Mourinho is] a guy who I wanted to play for since I was 10 years old, so now to have the opportunity to work under him again is great,’ Lukaku said.
‘We’ve been in contact. He explained to me how the club was like, what he expects from me.
‘You know, we were together under different circumstances. Obviously, the first time when I met him, I was still a young kid, and I was eager to play.
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‘And he understood my decision why I left at the time. I returned on the pitch. Now I’m 24. I’m a grown man.
‘He sees the maturity that has grown throughout the years, so I’m really thankful for the opportunity he gave me again.’