Manchester United bought him with expensive transfer cost but after that he did not value him properly and in my opinion he was used wrongly in the club, and was underrated by them. The transfer cost does not change how he was underrated in Manchester United, and he had to leave the club with not only disappointment but likely unhappiness.
You can't call a player who was signed twice for close to 100 million at his peak "underrated". If he were underrated, he would be bought for less. He was rated highly, which is why he was bought for that amount. He went to United, had a good first season and an average second season.
Despite having an underwhelming second season with Man United, Inter still made him their record signing with 80 million. He left for Inter and had two wonderfully successful seasons. Mind you, those were the only two seasons in his career where he scored 30+ goals in all competitions of the season.
He now has a 100 million move to Chelsea, where he was underwhelming again and was sold back to Inter for about 30 million. This doesn't sound like a player that was or is underrated.
In Belgium national team, people in many years talked about Hazard and De Bruyne while Lukaku was considered as not too much important and at lower level.
These players you mentioned were better and more important than him for the Belgium national team at their peak, so it's natural for them to be rated higher. Being the country's highest all-time goal scorer doesn't necessarily mean you were the best player. Giroud was the highest all-time goal scorer for France until Mbappe came along and surpassed him just a couple of days ago. Considering the kind of legends that have played for France, Giroud is nowhere near those guys.
Lukaku was not underrated. You could say he was treated harshly, especially at United, but he was not underrated.
