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League is very competitive with teams like Juventus and Inter Milan and AC Milan all getting better players. Because of this it is too early to assume Napoli will win, even if they sign Kevin De Bruyne. But he might not be at his peak his skills and experience would still be big help. And Napoli success will depend on their playing with rest of team. This is why upcoming season will be so exciting to watch.
Yes, in fact, these teams will do everything to win the current championship, it is certainly motivating for them, all the teams are doing their best to manage this thing and try to understand how the situation can evolve, it will certainly be difficult for Napoli but they can do great things compared to their direct rivals.