The Belgium international spent two seasons with the Red Devils before leaving for the Nerazzurri last year in a deal worth in the region of €65 million ($107.2 million).
United's attacking trio of Anthony Martial, Marcus Rashford and Mason Greenwood flourished in Lukaku's absence, while the former Everton man has thrived in Italy, scoring 34 goals in all competitions last season.
He was also named the UEFA Europa League player of the season after firing Inter to the final, where his own goal saw it lose 3-2 to Sevilla.
Lukaku's form has made a mockery of some criticism he received during his time in England, where it was sometimes suggested he was too slow and did not put in enough effort to lead the attacking line.
Speaking to The Times, Lukaku says he has proved those people wrong, although he insists the negativity around his performances was outside United rather than within.
"A year ago, when I was in England, I was 'lazy, I didn't run, I didn't do this and that'. Here, they call me the hardest worker in the room," he said. "If you look at me playing here and there, I mean, there are improvements, yeah, but the [same] Rom is still here.
"Stuff happens in football but it's something you need to explain to the next generation of kids, that you need to have full control over your destiny. Make sure when you go somewhere that every piece of the puzzle has to be right. Know what I mean? At the time, we saw the outcome, we saw how I did. And if you look at me now you see a different outcome, you see the full person, you see the full potential. You see what I could have been doing in England.
"From the first friendly game when I played against LA Galaxy, from that first game it was always 'Yes, but... Everybody in the club was so nice, from Ed Woodward to the owners, to Matthew Judge [transfer negotiator], to Jose Mourinho, to Ole [Gunnar Solskjaer] and his staff, to everybody in the canteen. They were so nice. But the stuff around it was just too much of a negative spiral for me.
"They will call me slow and I'm like, 'Slow? Me, slow? I cannot keep up with the fast pace game of Man U?' There were too many little things where I thought, 'This is not right'.
"If you call me slow I would not score a goal like I scored the second against Shakhtar Donetsk in the [UEFA Europa League] semi-final. If I was slow I would not have got the penalty like I did against Sevilla in the final. That's two years after. Are you going to say me at 27 is faster than I was at 25? That's what I'm trying to say. It's just little things. I was like, 'You know what? Eff this. I'm out.'"
Lukaku says Inter coach Antonio Conte, who tried to sign the player when he was with Juventus and Chelsea, has taken him to "another level".
He also believes the atmosphere within the Inter squad is strong, saying: "When I was in England, it was English players here, foreign French players there, foreign latino players there.
"It's all really like a mix up of the team, but at Inter, it is everybody together. Everybody speaks Italian first of all, so that's a good thing. There is one language. Everybody speaks Italian and everybody mixes at the table."
Lukaku holds no grudges against United, though, and has cautioned them against the panic-buying of a new attacking player given the development of Martial, Rashford and Greenwood.
He said: "I hear a lot of people saying, 'Let's buy another this, let's buy another striker, let's buy that'. If you buy another striker, what are you going to do with Anthony? Anthony is a good player. He's young. He's now being the number nine at a great club. Last year, he scored 23 goals. Give him a full season to confirm what he did last year instead of bringing in someone big to disrupt the chemistry in the dressing room.
"Then you have Mason's transition that is going to happen any time soon from right wing to number nine, and Mason is a killer. Like, I've seen him first-hand. He's got something.
"I remember Rashy came to my house [shortly before the Inter move]. We had dinner. Me, him and his older brother. I said to him, 'When I'm leaving, you're going to be the guy at United'. I'm so proud of him right now."