This match is important for them. AC Milan need to win to keep up their hope for the competition against Inter Milan and Serie A need it too. If AC Milan get a loss or draw and Inter Milan win their match, the Scudetto will almost be awarded to Inter Milan.
Napoli is in the very difficult competition for the 4th spot to Champions League. They are racing with AS Roma, Atalanta, Lazio and each match for those four clubs are important. They have 3 opponents that can take over their position on the table anytime.
The fact is Inter Milan is in great form now. If they lose their form, we will see a change on the point table. But that is very difficult for other teams. AC Milan is just 6 points behind Inter Milan. If they score 3 points per match from now on, they will probably be able to beat Inter Milan. But Juventus is 10 points behind. I don't think they can go before Inter Milan even if they win all the matches.
Inter Milan are in great form and their mentality is very strong. They are confident and never give up in their match. In this season, they score many single goal to end their matches with three points. Lukaku in this season is stronger and the drama with Ibrahimovic is an example for his mentality mature.