Coach Luciano Spalletti blasted Inter Milan's "sloppy and careless" performance as the fallen Serie A giants hopes of capturing this year's title faded with a 1-0 loss to Juventus on Friday.
Chinese-owned Inter, who finished fourth last season, had looked a serious rival to reigning seven-time champions Juventus with summer signings such as Radja Nainggolan, Matteo Politano, Sime Vrsjalko and Keita Balde.
But despite their spending spree Spalletti's side proved to be no match for Juventus, even with Portuguese star Cristiano Ronaldo not as omnipresent as usual on the night for the Turin giants.
Inter sit third, but are now 14 points behind Juventus, and risk losing ground on second-placed Napoli, who are three points ahead of Spalletti's side, and play lowly Frosinone Saturday.
City rivals AC Milan, just behind in fourth, could also close the gap when they host Torino.
Mario Mandzukic's diving header after 66 minutes was enough to settle the Derby d'Italia with Inter's Roberto Gagliardini missing a golden chance after 30 minutes to help Inter to their first win in Turin since November 2012.
"We managed the game in the right way for long spells, but we're still too careless at times and sloppy," said Spalletti.
"It would have made a world of difference had we taken the lead, but we misplaced too many passes, we made too many banal and cheap errors.
"There were situations where we didn't interpret or read them property while Juventus didn't do anything wrong," said Spalletti.
"Unfortunately, Juve are smart and we are naive. They have a way of wasting five minutes after a foul."