Two AC Milan players - Gianluigi Donnarumma and Jens Petter Hauge - who played against Celtic on Thursday have tested positive for Covid-19.
Goalkeeper Donnarumma played the full 90 minutes at Celtic Park while substitute winger Hauge secured the 3-1 Europa League win with a late goal.
The Italian club say three other members of their staff have also gone into isolation after positive tests.
It comes prior to the Serie A leaders' home game against Roma on Monday.
Milan say the results followed swab tests conducted on Sunday evening and point out that further tests of players and staff carried out on Monday had produced no further positive results.
Of those who tested positive on Sunday, they added: "Whilst they are all asymptomatic, they have immediately entered isolation in their homes and the relevant local health authorities have been informed."
Milan play their second game in Europa League Group H at home to Sparta Prague on Thursday while Celtic travel to take on Lille.
Celtic have themselves been without France Under-21 striker Odsonne Edouard and Israel defenders Hatem Elhamed and Nir Bitton for three games after they tested positive while on international duty.
- New - The Rap Game UK: The search for a new British rap star is on
- MOTDx is back: The very latest on fashion, music and culture from the Premier League