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According to the odds the bookie placed Barcelona as the favourite with odds @1.84 and AC Milan as the underdog with odds @3.55 but if we see from head to head statistics both teams has been met once on 2018 at that time AC Milan can able to beat Barcelona with slim score 1-0 and indeed Barcelona recent performance looks very impressive that they had showed good progress by humiliated Real Madrid and for this match too probably Barcelona win could be good pick and @1.84 is decent odds
Barcelona are obviously the favourites, even if they didnt play in the cup they won something (La Liga) while Milan didnt do much this season.
Lets say that he also had a bit of bad luck (Milan) many injuries and a lot of bad luck. Never as much as Juventus but they suffered.
Certainly Barcelona are the favorites without a doubt, they have a history and a squad of champions that already works and allows them to play well together. Teams like Milan are teams to rebuild because they have something wrong, some notes that are out of tune, I don't know if you can understand me.
this is my opinion