So now we know. After the breathless drama of Monday night all - all! - that's required of Scotland on Tuesday is a win against Croatia by a single goal and they enter the rarefied air of the knockout stages of a major championship for the first time.

Some of the greatest names who ever put boot to ball have tried and failed, but this group can change a decades-old narrative. No relying on other teams to do them a favour, no frantic checking on updates from other matches in other grounds in other cities. A victory and it's done. Nirvana here we come. Maybe.

Of course it's way, way easier now than it ever was for the Bremners and the Dalglishs and the Laws and all the other Scottish immortals who fell short, but these bloated tournaments are what they're calling progress at Uefa these days.

Whatever it is, Scotland have a chance to take advantage. One battle-hardened veteran of the Tartan Army sighed heavily when contemplating this hopeful scenario last night. "Aye," he said, with the slightly haunted look of a man who was thinking about the ghosts of campaigns past. "What could possibly go wrong?"

Well, Billy Gilmour is already out, so there's that. As he tries to get over the enormous disappointment of missing this game we're guessing Gilmour won't take any comfort from knowing that the reaction to the breaking news of his positive Covid-19 test was nationwide hyperventilation. Such was the excellence of his performance against England, he has gone from a star of the future to a star of the here and now.

A few days ago we had no cast iron idea about how quickly Gilmour would take to the white heat battles of international football. Now we know he takes to them like a Tartan Army member takes to a fountain of Fairy liquid. Scotland's chances are diminished without him. Croatia - even the great Luka Modric - will be cheered by his absence.

Scottish footballing fatalism covers vast amounts of terrain, however. People scarred by previous mishaps might be minded to think that the Gilmour episode is just the beginning of heartache. Is Kieran Tierney waking up with a tightness in his calf? Is Andy Robertson tripping over a cat? Is Che Adams about to have a nasty accident with a fried egg, Kirk Broadfoot style?

Much of Monday was taken up with chat about how Gilmour could have got coronavirus, why none of his team-mates had to isolate as close contacts, why an Instagram video of Gilmour, Robertson and John McGinn playing table tennis on Sunday night was mysteriously deleted as the Scottish FA attempted to reassure everybody that none of the other Scotland players would be ruled out as none had strayed that deeply into Gilmour's personal space.

There was talk, not of formations or selections but about Uefa's chief medical officer and officials from Public Health England and manifests and protocols. Steve Clarke was peppered with questions about procedures rather than players. This kind of virus story was always going to erupt and cause mayhem during the tournament at one point or another, but the turbulence had to be felt in Scotland first, didn't it?

Clarke is the kind of man you want in a situation like this. Big and strong and not one to mope. Coronavirus could have denied him the services of half a dozen of his players and, publicly, he wouldn't have blinked. He'll have heart-felt sympathy for the deeply unfortunate Gilmour, but his transition from sorrow for a fallen player to picking his successor would have taken about as long as the light to go out when you flick the switch. 'Billy's out, who's coming in?'

It's probably going to be Stuart Armstrong - a highly acceptable stand-in with huge experience. Or, perhaps, Scott McTominay will step back into midfield with Liam Cooper returning to the defence. Cooper was on media duty yesterday. A clue or a curve ball?

The blessing about Monday's hubbub is that Public Health England, having pored over the detail, reckon that no other Scot needs to be put in a room for 10 days. If you want to look for positives amid the Gilmour gloom, that's actually quite a major one.

And here's another. Croatia are in ropey condition and they're getting pelted with flak by their media and fans at home. They haven't won any of their past four games, one of them against Armenia. In the past 18 months they've played 15 times and have won on only four occasions.

Sure, most of the defeats have come against a certain level of opposition - England, Belgium, Sweden, France twice and Portugal twice. They've also lost to Slovenia and drawn with the Czech Republic, Armenia and Turkey. Their victories have come against Malta, Cyprus, Sweden and Switzerland.

The form line offers hope to Scotland. Listen to Modric: "We have to change a certain chip in our head, all of us, from the first to the last player." Listen to Ivan Perisic: "We need to be more confident, we need to have faith in ourselves. We haven't had that in the last five or six games and we have to bring it back. We need to improve at least 25% or we are going home." And Modric again: "It's as if some kind of uncertainty has crept in. Our fans are rightly dissatisfied. We as players are dissatisfied."

Those guys are World Cup finalists. The team that drew 1-1 with the Czechs had five survivors from the side that lost to winners France in Russia three years ago. But there's something missing. Are they now over the hill? Has their time been and gone? These are the debates that are taking place from one end of the country to the other. Scotland's not the only place that has a mission to fulfil at Hampden. The hunger for qualification is every bit as great among these Croatian players - perhaps raging against the dying of the light - as it is for the Scots who are playing for a bright new dawn.

It's hard to see Clarke making any changes beyond the one brought on by Gilmour's absence. He'll go with the same players and will hope for the same intensity that he got from them at Wembley. Can they reproduce that kind of concentration and discipline and physicality and add a goal to boot?

It'll be hard to replicate the controlled fury they had in London. The emotion of the night was enormous and it was channelled brilliantly. Only that kind of hustling and harrying will do. Only that kind of in-your-face belligerence will bring them close to their dream. A draw this time is no good. They must score. The Scots are the only side in the entire tournament who have yet to register a goal.

Clarke believes in their capacity to get the job done. He cites the chances they've created and the bit of luck that has eluded them. Gilmour's loss might be used as galvanising factor, a useful source of motivation and edge. A Hampden tie to guarantee a passage to the next round. This is it, the moment of truth. Their own stadium, their own people, their own destiny in their own hands.