The Chelsea midfielder’s positive test result comes just three weeks aftter similar struck down John Fleck and ruled the Sheffield United midfielder out of the preparatory camp and warm-up games with the Netherlands and Luzembourg.
Fleck also sat out the matchday squad against Czech Republic and now, for the second Hampden game, there will be another enforced absence in the middle of the park following Gilmour’s news.
It’s a wicked blow to the midfielder who played so well at Wembley, and to the manager’s plans after he got the game plan so right in the Auld Enemy clash.
But now it’s up to the others to take Scotland through to the last 16 for the first time and assuming there are no further ramifications from the latest positive case in the camp, there are plenty of other candidates ready to step up against Croatia in a must-win match.
Even then, progress is not guaranteed but is this the team Steve Clarke will stand by to give the Tartan Army the best possible chance of seeing another match (at least) this summer?
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1. David Marshall
The Scotland goalkeeper was solid at Wembley after being caught out by the Czech Republic. No reason to suspect he won't retain his place once more - and will surely keep his bearings on the Hampden pitch this time around. Photo: Stu Forster
2. LWB - Andy Robertson
Only one way the captain will miss this one - if he's identified as a close contact of Billy Gilmour. But word form the Scotland camp suggests there's no concern and after two excellent showings so far in the tournament - a third is crucial if Scotland are to have any chance of progress. Photo: CARL RECINE
3. LCB - Kieran Tierney
One of the what-ifs of the tournament. Assuming there are no ill effects from England the Arsenal man will be included - and vital again. Missed the Czech Republic game and was a big miss, needed more than ever to cause Croatia problems with his driving runs and keep them out of Marshall's area. Photo: ANDY BUCHANAN
4. DC - Grant Hanley
Defensive centre-point has been reliably solid in both games - and particularly stood out at Wembley. No nonsense at the back is the sort of attitude Steve Clarke seeks in his role in the central three, and the Dumfries man delivers. Photo: FRANK AUGSTEIN