The Turin giants put their foot on the gas after a balanced first half with Gennaro Gattuso's Milan crumbling with two goalkeeping errors from young 'keeper Gianluigi Donnarumma and a Nikola Kalinic own goal gifting the title to Juventns.
But after the interval defender Benatia turned goal scorer heading in off a Miralem Pjanic corner on 56 minutes, for the first goal conceded by Milan in the Italian Cup this season.
Gattuso, in his first season in charge of Milan after taking over in November, also took some of the blame for the result, though he also said "the result does not reflect the performance" of his team.
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"That's football", he said. "Now for 180 minutes we have to be Milan, all together". We all make mistakes and today we can't judge Donnarumma.
However, the Old Lady broke the deadlock 10 minutes into the second half as un-marked Mehdi Benatia headed past a helpless Donnarumma. Things changed from the 50th on as Juve scored 4 unanswered goals (Benatia scored 2, Douglas Costa scored one and Kalinic scored an own-goal).
Five minutes later, Donnarumma allowed a low shot from Costa to slip through his hands and into the net and the hapless goalkeeper then failed to hold a header, allowing Benatia to snap up the rebound.