Lukaku for Belgium is still not at the right level for a top team. Just doesn't affect the game anywhere close to enough, doesn't make enough runs and is a big part of the reason why Belgium, despite having the likes of Hazard, De Bruyne, Alderweireld and Vertonghen just struggle with him (and pretty frequently look a more fluid side with someone else up top, though whoever goes up top is usually not as reliable of a scorer against the smaller nations which is the majority of international football).
I think he'll be decent in the Serie A, but Inter will be downgrading anyway going from Icardi to Lukaku. Surely if they go all out for Lukaku then they will sell Icardi? Not that he's perfect for us, but I do think he's far more suited to a top team simply because his natural striker instincts like his off the ball movement, his ability to use his body battling with defenders, his ability in the air and his goalscoring ability are all far better than Lukaku's. Would instantly take him tbh, but we still need a proper right winger to provide the delivery into him.
Either way, Icardi or no Icardi being part of the deal, I would let Lukaku go to Inter. He's not good enough for United, he doesn't like being a bench player and the wages + fee he could bring in is not something that should be on our bench when our starting 11 is so mediocre.