The Turin club wrapped up an eighth successive league title last weekend but found themselves behind early on to a wonder-strike from Inter midfielder Radja Nainggolan.
The 19-year-old got his first ever Champions League goal in the 70th minute in Rome, adding a second six minutes later to inflict a first defeat on Porto in this year’s top-tier European competition.
Marotta arrived at the Chinese-owned Italian club from Juventus this month after an eight-year spell during which the Turin giants won seven Serie A titles and four Italian Cups.
The Argentinian pounced on a mistake by goalkeeper Gianluigi Donnarumma to nod home in the second minute of added time after an otherwise forgettable match, leaving Gennaro Gattuso’s Milan without a win in the last five league derbies.