David Marshall admits the chances of Scotland qualifying for the 2020 European Championships are "slim" after the 2-1 defeat by Russia at Hampden Park on Friday.
Steve Clarke's side took the lead after just 10 minutes when midfielder John McGinn scored his first international goal but Russia scored either side of half-time to take all three points.
After just two wins in five fixtures, Scotland are in fourth place in Group I with six points while Russia are second with 12 points, three behind leaders Belgium who visit Hampden on Monday night, live on Sky Sports Football.
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The Nations League semi-final play-off next March offers Scotland another way into Euro 2020 but when asked about the chances of getting there by the traditional route, the 34-year-old Wigan goalkeeper said: "It is obviously slim.
"We know that, we are realistic about it. There is half the campaign gone and we have to go and win on Monday night, we know that.
"It is a massive ask at this stage but you feel better in a couple of days after you get a few days training in and we will go again on Monday.
"There is a lot of football to be played. We need to get our heads right. There is still five games to go in the group.
"We know how slim it is but we need to go and win a big game.
"We obviously know Belgium really well. We will have a game plan which may change because it has turned into a game we need to go and win now. I am sure we will be ready.
"Football changes very quickly. We were in the ascendancy and came off it. So we will go and give it a go on Monday.
"We know what the crowd is like here [at Hampden] if we get going and if we can get a start on Monday like we did against Russia hopefully we can maintain it. But we are under no illusions."
Like most of the Scotland players, Marshall was at a loss as to how the home side failed to build on their early good play and goal.
The former Celtic, Cardiff and Hull goalkeeper said: "It was a big game and we needed the points. They are a good team but I don't think we played that well.
"It was just a disappointing result. Our game plan was to get after them and it worked really well.
"It was a perfect start for us but the game just seemed to change. They got a lot of possession and the goal before half-time was important for them."