Sarri had made it to Juventus’ training ground today but was unable to take training.
Last week, the 60-year-old was suffering from the flu and the symptoms failed to subside, causing him to miss Juve’s friendly win over Triestina.
And on Monday at the club’s training complex, tests confirmed that Sarri, who left Chelsea earlier this summer, had pneumonia.
A club statement read: “Maurizio Sarri, after having observed rest over the weekend, was at the JTC Continassa 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.”
Juventus face Parma in their season-opener on Saturday but it is not clear as to whether Sarri will be able to take the reins from the touchline.
Sarri admitted recently that he smokes close to 60 cigarettes per day, something bound to be a detriment to his health.
“It’s 60 a day, probably a few too many,” he revealed last week.
“I don’t particularly feel the need to smoke during games, but straight afterwards it really is necessary.
“I had back problems tormenting me for weeks, but I feel much better now.”
Last season at Chelsea, the Italian struggled to impose his style on a Blues side that finished third in the Premier League.
Sarri did, however, manage to win his first trophy as a manager, when he picked up the Europa League following a 4-1 win over Arsenal.
But his time in west London came to an end after mutually agreeing to leave the club for Juventus, while Chelsea brought in Frank Lampard as a replacement.