The 26-year-old striker spent just two seasons with the Red Devils after being snapped up from Everton for £75million by then Red Devils boss Jose Mourinho.
He registered 42 goals in 96 appearances, but still failed to convince many that he was the man the lead the line at Old Trafford.
With Mourinho moved on and Ole Gunnar Solskjaer appointed in his place, Lukaku found himself slipping behind the likes of Rashford and Anthony Martial in the pecking order.
That contributed to the decision to push for an exit, which was granted when he headed to Inter Milan for an initial fee of £73m over the summer.
The Belgian has had a positive at the San Siro, despite some distractions, and his representative believes the best decision was taken for all concerned.
Federico Pastorello told Sky Sports: “He was happy at Man United. He liked the fans a lot.
“For sure, Man United is, I don’t want to see most important, but in the top three clubs in the world. He was happy about that.
“But I think he wanted to prove himself in another championship. He needed another challenge.
“The relationship with Ole at the end was good.
“There were maybe some issues tactically because he’s a No.9 striker and maybe sometimes in that position the coach preferred another amazing player that was Rashford.
“So the fact he moved was not down to some particular issue with Man United, he wanted to try a new experience.”
Lukaku is already into double figures for the 2019-20 season, with 11 goals recorded in just 17 outings for Inter, who are top of the Serie A table under Antonio Conte.