The 18-year-old had been expected to move directly to Chelsea after the club agreed personal terms and a fee with Atalanta last week, with Ahanor already given permission to travel to London for a medical.
Instead, he will officially join Crystal Palace on loan from Atalanta this season before completing the switch to Chelsea once that spell concludes.
The structure exists because of a quirk in Chelsea's own summer business: with Axel Disasi having already joined Palace on a separate season-long loan with no option or obligation attached, sending a second player to the same club required Ahanor's move to be routed differently, via a loan from his current club rather than from Chelsea itself.
A product of Genoa's academy, Ahanor has already won two senior caps for Italy, making his debut earlier this year, and made 28 Serie A appearances for Atalanta last season, including his Champions League debut in September.
The arrangement gives Ahanor a season of Premier League football under Pierre Sage before he eventually moves to west London, continuing Chelsea's pattern of parking highly-rated young defenders at other clubs to build senior experience.
For Palace, it adds further depth to a defence already being reshaped following a series of high-profile departures from last season's trophy-winning squad.
Chelsea's willingness to defer Ahanor's arrival by a full year underlines a broader strategy of stockpiling talent through pre-agreed deals structured around loan spells elsewhere, a model the club has used repeatedly in recent windows to secure highly-rated prospects without immediately adding to a crowded first-team squad.
For Ahanor himself, regular football in the Premier League - even at a club other than his eventual destination - represents a more realistic path to development than fighting for minutes behind Chelsea's established options at centre-back.