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Lukaku deserves to be signed by Inter Milan because his career is very bright there. Lukaku sees that his performance in inter Milan looks great, which is why he also preferred to stay in the team even when he was offered a significant sum of money to go Saudi Arabia. I believe that Inter can do anything to obtain a good striker who will improve the team,They will want to avoid having a terrible performance because they are also participating in the champion league.
In addition, Onana has put a lot of effort into helping inter Milan, and he deserves to be recognised as the team's best goalkeeper. However, I don't believe that will cause inter Milan too many problems because, regardless of the situation, inter will have a quality goalkeeper in place before the league season begins. Finding a quality keeper may be challenging, but selling Onana won't have a negative impact on inter Milan, in my opinion.
If you don't think Andre Onana's departure has affected Inter's performance, then they shouldn't have to look for a suitable replacement for him. So I can say that your assumption is not correct because a good goalkeeper and consistent form will help the team to gain stability in defense. So, if Inter sells Onana, then they must find a suitable replacement so that their defense is not weak.
Apart from having good defenders, midfielders and strikers, every team should also pay attention to the position of the goalkeeper. It's not easy to get a goalkeeper with good quality, so if team has one, then you will find that team won't concede goals easily.