The Scotland midfielder joined in August from Manchester United.
Former Italy coach Sacchi told La Gazzetta dello Sport: "Napoli, in my opinion, is the team that has made the most evident improvement. Antonio Conte is a master who loves his job so much and transmits this passion to his players. In a short time he has been able to create a group and the first place in the standings is no coincidence.
"Conte demands a lot in physical terms from his players and hammers them from a psychological point of view. He is, in short, the added value of this team that, after the Scudetto won with Spalletti, has had a negative season.
"What impressed me about Napoli is the team's compactness, its ability to defend with eleven and attack with eleven, always with the right distances. This means that Conte's teachings have been perfectly understood. The midfielders move in sync and the thrust of the wingers is constant and always creates danger for the opponents.
"I'm admiring the inclusion of McTominay, an element that is proving to be very valuable for Conte's game: he helps in the containment phase and has flashes of a true trequartista.
"Even mentally I noticed an evolution: the defeat against Atalanta could have created problems and instead Napoli responded with an excellent performance against Inter. A sign of maturity. The Scudetto will be played until the end with Atalanta, Inter and Juve."