The Old Lady have chosen ex- Italian national team boss Spalletti as their manager for the remainder of the campaign.
The experienced coach steps in for Igor Tudor, who was let go post the Lazio setback last weekend.
Tudor arrived at the club in March as Thiago Motta's replacement but left after seven months after an eight-game winless streak.
Spalletti guided Napoli to the Serie A crown in 2022-23 then moving on to lead the Italian national team, who had failed to qualify for the 2022 World Cup with Mancini at the helm.
He helped them qualify for Euro 2024 but were eliminated in the round of 16 by Switzerland.
However, Spalletti was sacked by Italy in June after a World Cup qualifying defeat by Norway, putting Italy in danger of missing a third straight World Cup.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and lifted the Russian league trophy twice while at Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus in the midweek 3-1 triumph over Udinese which left them seventh in Serie A - trailing leaders Napoli by six points after nine matches.
In the Champions League, they sit 25th in the league phase, after recording one defeat and two ties in the initial three fixtures.
