Sarri missed Juve's friendly win over Triestina on Saturday due to "the persistence" of flu, which he developed last week.
He returned to the club's training base on Monday but was unable to oversee preparation work on the pitch due to his illness.
The club conducted further examinations and Sarri has now begun a course of treatment.
A statement released by the club on Monday read: "Maurizio Sarri, after having observed rest over the weekend, was at the JTC Continassa [Juve's training facility] today where he coordinated the work of his staff.
"Unfortunately, he could not conduct the training on the field due to the persistence of his flu, which struck him over the course of last week.
"In the late afternoon, he underwent further tests that confirmed he has pneumonia, for which specific therapy has been prescribed.
"The coach has authorised the club to communicate his state of health."
Juve begin their Serie A campaign against Parma on Saturday. It is unclear whether Sarri will be in a position to take charge of the team at Il Tardini.