A specialist consultation in Barcelona confirmed the need for surgery to repair the medial meniscus root in the defender's right knee, with the operation scheduled for the coming days. The absence is a significant blow for Napoli, who lose one of the key figures in their defence for the early part of the season.
Without Buongiorno, Massimiliano Allegri has been forced to rethink his defensive options during the opening days of the club's training camp in Dimaro, trialling a pairing of Rafa Marin and Amir Rrahmani that currently appears to be his starting solution.
Sam Beukema, also managing an injury of his own but still present in camp, offers a further option alongside Marianucci and Obaretin, the latter having been kept on with the squad specifically to cover for Buongiorno's absence despite being expected to leave the club this transfer window.
Elsewhere in Naples, Napoli's goalkeeping situation could also be about to shift. According to Rai's Ciro Venerato, the club are exploring a possible swap deal that would send Vanja Milinkovic-Savic, signed for €22m just a year ago, to Tottenham in exchange for Guglielmo Vicario.
Alex Meret would be restored as first choice under Allegri following Antonio Conte's departure, with the details of any Napoli-Tottenham exchange expected to become clearer in the coming days.
Amid the injury and market news, forward Alisson Santos took questions from supporters during the club's training camp in Dimaro.
Asked what he felt when he learned Napoli wanted to sign him, Santos said: "I was very happy when I had the chance to come to Napoli. I wanted to come here straight away. I know it's a huge club and for me it's a dream to be here."
Asked about his goal against Roma and his connection to the number 27, he added: "I was very happy when I scored my first goal in the Napoli shirt. It's a really big feeling. The number 27 is because it's my birthday."
Buongiorno's injury adds further complication to a defence already being reshaped following Antonio Conte's exit, with the Italy international having been one of Allegri's most trusted options heading into pre-season before the setback in Barcelona.
Napoli's medical staff had been monitoring the issue closely in recent weeks, and the club's official statement confirmed there was no viable path to a conservative recovery, leaving surgery as the only option to fully resolve the problem.