I don't think Lukaku will be considered as important for Milan after he was important for rival club Inter. I don't think Milan will really wants to sign Lukaku from Chelsea as striker, even if they have the money to complete a transfer. Lukaku also doesn't seem to suit Milan's style of play, he would rather be at Inter for a long time after he was known as a dangerous striker at Inter for several seasons.
Even if Chelsea have to sell him, it is very likely that Lukaku will not be sold to Milan. There are a lot of better players than Lukaku, so Milan shouldn't be too interested in signing Lukaku.
The thing about Lukaku is that you don't know the team that might fit his style of play. You might think he won't do well in Milan but the opposite might be the case.
Lukaku is not a bad striker. I feel his major problem is confidence. If Lukaku has the confidence he can be a pretty deadly striker.
The mistake he made was leaving Inter for Chelsea in the first place. He should have just remained at Inter when he was one of the hottest strikers in the planet. Now that he's at Inter he doesn't perform much. Dzeko was even higher up the pecking order than him.
I believe Lukaku will do great at AC Milan though.