It means that Inter value Lukaku highly and United don't want to lose Lukaku for cheap. Both of them agreed on the deal so that's that. If Lukaku can't hit his top form again, I don't think it's possible for any team to sell Lukaku for more than half of his price right now.
Would be interesting to see how he performs in Inter. Hopefully, he can get his best form again and compete with Ronaldo so that the Italian league would be interesting to watch.
Lukaku is a good player, especially if we look at him in physical strength. In contrast, he does not have good speed and this one somehow limits chances to succeed in Premier League. However, this disadvantage will dissappear when Lukaku plays in Serie A, that does not aks for highly speed to outperform other players. In Serie A, Lukaku can take advantage of his strenght, his height and I believe that he will take official solid posiition in Inter Milan Squad soon.
In general, this season, I still don't think that there are any clubs that can beat Juventus FC. I am a big fan of AS Roma, but honestly there is no chance for AS Roma to join the top three this season, in my opinion. Napoli might still be the biggest competitor for Juventus, but they will hardly to beat Juventus.
Kostas Manolas is a good contract for SSC Napoli, but Juventus is outstanding team in Italy, for now.