Milan on Thursday revealed Olivier Giroud has tested positive for coronavirus.
Giroud opened his Milan account with a double in a 4-1 Serie thrashing of Cagliari on Sunday.
The 34-year-old striker has now been forced into isolation after returning a positive COVID-19 test.
"AC Milan announce that Olivier Giroud tested positive for a molecular test carried out at home," the Rossoneri said in a statement.
"The local health authorities were immediately informed. The player is fine but must remain in fiduciary isolation.
"It should be noted that Olivier has not had contact with the rest of the team since the championship game against Cagliari."
Giroud joined Milan in a reported €2million (£1.7m) deal from Chelsea in July.