The Old Lady have appointed ex- Italian national team boss Spalletti as their manager for the rest of the term.
The 66-year-old takes over from Igor Tudor, who was dismissed post the Lazio setback last weekend.
Tudor had joined the club in March filling the shoes of Thiago Motta but exited following a seven-month stint following a string of eight matches without a victory.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 then moving on to lead the Italian national team, which missed out on the 2022 World Cup with Mancini at the helm.
He secured their spot in Euro 2024 where they were knocked out in the last 16 by Switzerland.
However, Spalletti was sacked by Italy in June post a qualifying defeat against Norway, as Italy faced the possibility of not making a third World Cup in a row.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and secured two Russian Premier League titles with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla during the 3-1 victory over Udinese on Wednesday moving them to seventh position in the league - trailing leaders Napoli by six points after nine matches.
They are 25th in the league phase of the Champions League, with one loss and two draws in their first three games.
