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.