The 60-year-old Italian was suffering from the flu last week but Juventus confirmed that his condition was more serious.
Sarri was at Juventus' training center on Monday but his condition meant he took no part in the day's operations.
"Unfortunately he could not conduct the training on the field due to the persistent flu, which struck him over the course of last week," a club statement said on Monday. "In the late afternoon, he underwent further tests that confirmed he has pneumonia, for which specific therapy has been prescribed."
Sarri reportedly received treatment before the traditional preseason tune-up against the club's youth team last Wednesday. His condition did not improve and he was unable to attend the 1-0 friendly win over Serie C outfit Triestina on Saturday.
In June, Sarri confessed to smoking 60 cigarettes a day. "I don't particularly feel the need to smoke during games," he told Football Italia. "But straight afterwards it really is necessary."
Juventus, chasing a ninth consecutive domestic league title, kicks off its campaign at Parma on Saturday but might be without its new manager who replaced fellow Italian Max Allegri over the summer.
Sarri returned to Italy after a single season in England where he steered Chelsea to third in the Premier League and lifted the Europa League after beating Arsenal 4-1 in Baku.
Despite the seemingly impressive debut season in England, Sarri was lured back to his homeland where he last managed in 2018 having taken his native Napoli to second in the league. He was replaced at Stamford Bridge by former Blues midfielder Frank Lampard.