Star forward Ronaldo and midfielder Blaise Matuidi took to the Turin-based club's medical centre on Saturday to undergo physical testing ahead of the new campaign.
New boss Sarri, who succeeded Massimiliano Allegri last month, took charge of training for the first time on Wednesday.
Deciding on Ronaldo's best role in his preferred 4-3-3 system will be a key item on the Italian's agenda.
The 34-year-old former Real Madrid man played both centrally and out wide on the left in his first season in Turin, contributing 28 goals and 10 assists across all competitions.
One member of that Ajax side, Matthijs de Ligt, is on the cusp of moving to the Allianz Stadium after the clubs reportedly agreed a fee of around €70m.
Portugal defeated Netherlands 1-0 to be crowned the tournament's inaugural champions.
Juventus have been busy in the market outside of De Ligt’s reported capture, splashing out on Luca Pellegrini and Merih Demiral, as well as landing Aaron Ramsey, Adrien Rabiot and former goalkeeper Gianluigi Buffon on free transfers.
Meanwhile, Leonardo Spinazzola, Emil Audero, Stefano Sturaro, Rogerio and Luca Marrone are just a few of the players to have been moved on.
Cristian Romero has also been signed from Genoa, but he will return on loan for the coming season.