The Chelsea starlet put in a man-of-the-match display against England at Wembley on Friday evening, and was widely expected to feature again in the Tartan Army's final group match at Hampden Park, with a spot in the last 16 up for grabs.
After an update on Monday morning, however, the 20-year-old will not be available for selection.
A statement from the SFA said: "The Scottish FA can confirm that a member of the Scotland national team playing squad, Billy Gilmour has tested positive for Covid-19.
"Having liaised with Public Health England since the positive test was recorded, Billy will now self-isolate for 10 days and therefore miss tomorrow's Uefa Euro 2020 Group D match against Croatia at Hampden."
Despite Gilmour playing on Friday and being part of the Scottish squad’s training bubble, no other players are understood to be have been affected or identified as close contacts, meaning the remainder of the squad will be available.
We’ve gathered the best reactions from supporters below…
@SunScot tweeted: “Gutted for Billy and wishing him all the best. The rest of the team can still do this!”
@DanMcG97 tweeted: “A massive blow but we can still get a result. I hope Clarke picks the same team, except bring in Turnbull for Gilmour”
@celticfitba tweeted: “My god, how about that for bad luck? What a rollercoaster ride the wee man has been on the last few days. Get well soon Billy”
@dylboy72 tweeted: “This is the most Scotland thing ever”
@mfclauren tweeted: “Is this a joke?”
@nowmrsflynn tweeted: “Poor lad. All of Scotland is gutted. What an achievement after a single play out for the national team.”
@MichaelMacLeod1 tweeted: “Back for the semi final and final then”
@Snid1989 tweeted: “Just let him play, no one gets near him anyway”
@Nook0201 tweeted: “Does Phil Foden have to self isolate too? He was in the wee man’s back pocket all Friday night, surely a close contact”