Even if Spalletti had stayed (or any other coach had come), repeating the championship with Napoli would be a very difficult task. Napoli has been one of the big four in Serie A for a long time and is almost as good as Inter, Milan and Juventus in terms of finances and opportunities, but still in this campaign they are eternal underdogs and can take the title either against the backdrop of a decline in the rest or due to a phenomenal series victories like last season. But it’s hard to expect that they can show two such seasons in a row.
The performance of Napoli began to reduce towards the end of the last season. The club began to struggle to win games they had to wait longer to win the trophy because they couldn't secure a win or draw. Luciano Spalletti is not a magician but he would have managed the club better than the current coach. The reason is that he already understands Napoli players more than Rudi Garcia. The new coach is struggling to get used to this player and it is affecting the performance of the club. Napoli is facing stronger opposition this season than last season when the top clubs underperformed.
Garcia will influence the team in any case because he will use his strategy, he will have his training regime and other habits that can affect the team. Whether this will bring the team a better result or not we will see, but without Spalletti we will most likely see a different Napoli. It is normal that the team may change with the start of the new season. By the way, I can say that Inter worked very well in the off-season and they have an excellent chance of winning the Scudetto this season.
The management of Napoli has to be patient with the new coach. I don't think they should expect him to win the league just like that. He needs time to build a team that understands his pattern of play. The team still retained some quality players who can still compete in different competitions.