For much of the summer it had appeared that the 24-year-old was bound to leave Everton for Chelsea, only for United to swoop in and gazump his former club.
Mourinho was in charge when Lukaku was given the green light to leave Stamford Bridge, but three years on he now sees the striker as a key piece of the jigsaw at Old Trafford.
United forked out an initial £75million for the Belgium international, whose mind was made up by his manager’s plans rather than the persuasive powers of close friend Paul Pogba.
“We were just on holiday together,” Lukaku said of Pogba. “At the end of the day I had to make my own choices.
“It was the conversation I had with the manager that really convinced me the most, the plans and how he wanted to rebuild the club and he wanted me to be part of it.
“I am grateful for the chance.
“For me, it’s all about winning,” Lukaku said in Washington. “I want to win and now I’m at a club where winning is the most important thing.
“It’s something inside of me, so I just want to win and I want to help my team-mates deliver wins game in, game out. It’s always been like that. I’ve been a guy who is working to be a pro since I was 11 years old, so I just have a goal to win as many trophies as I can.
“At the end of the day, to play for a club like Manchester United is something that I’ve always wanted. It’s an opportunity I have now and I want to grab this opportunity.”
Lukaku is enjoying working with Mourinho again — “I believe in his methods” — and immediately felt “something special” upon joining the group.
“I’m a goalscorer, I’ve always scored goals throughout my career. Also in Europe, when I played in the Europa League, my record is really good. Now I think the Champions League is the next stage where I have to prove myself and the club wants to do well in it as well.”
Asked if he was at the level of Robert Lewandowski and Cristiano Ronaldo right now, he quickly replied: “No, no, no - I’m far from that level. But it’s the level I want to aim for. But it’s all about the team, I want the team to win trophies and I’ll do everything that I can in my powers to make sure the team wins.”
Lukaku has big boots to fill following the exit of injured top scorer Zlatan Ibrahimovic and the club’s all-time leading marksman Wayne Rooney.
Anthony Martial gave Mourinho a timely reminder of his undoubted quality, producing a mesmerising assist as United beat Real Madrid in a penalty shootout early on Monday in Washington.
A little more than a fortnight before their Uefa Super Cup meeting in Macedonia, the sides met at Levi’s Stadium, home of the San Francisco 49ers, in the International Champions Cup.
United fought out a 1-1 draw with Madrid in Santa Clara, where Jose Mourinho’s men triumphed 2-1 on penalties as a string of terrible spot-kicks brought the friendly to an entertaining end.
Meanwhile United defender Eric Bailly has received a three-match European ban after he was sent off in May’s Europa League semi-final against Celta Vigo.
The Ivory Coast international was dismissed after putting a hand into John Guidetti’s face, with Celta defender Facundo Roncaglia also dismissed for raising his hand to the United centre-back in retaliation.
Bailly missed the Europa League final victory over Ajax through suspension but has been handed an additional two-game suspension by Uefa’s control, ethics and disciplinary board (CEDB).
It means Bailly will be unavailable for United’s Super Cup clash with Champions League winners Real Madrid and also the first match of the Red Devils’ return to Europe’s elite club competition.