Juventus revealed on Monday that Maurizio Sari had pneumonia five days before the start of the new season.
The 60-year-old, known as a heavy smoker who took over in June instead of Massimiliano Alegre, suffered a cold last week and missed a friendly on Saturday, Juventus said.
Sarri, who left Chelsea after a season to return to Serie A, attended the training on Monday and "coordinated the tasks of his assisting team" but did not oversee the training on the pitch, the club said.
Juventus did not say whether Sarri would lead the team in the opening match of the new season at Parma on Saturday.