John Souttar has been called up to replace the injured Grant Hanley while Ryan Fraser has withdrawn from Scotland's squad to play against Moldova and Denmark over the international break.
Souttar, who has earned three caps for the national team to date, has impressed for Robbie Neilson's Hearts so far this term, making 11 appearances with the Edinburgh club third in the Scottish Premiership.
Scotland boss Steve Clarke has been handed some unfortunate injury news prior to his team's two crunch qualifiers, with Ryan Fraser and Grant Hanley having been forced to drop out.
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Hanley's muscle issue was confirmed by Norwich earlier this week and the centre-back missed his side's 2-1 win over Brentford on Saturday.
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If Scotland beat Moldova in Chisinau on Friday, they will have secured a place in the play-offs to qualify for the Qatar 2022 World Cup.
A draw would see Clarke's side finish second in Group F if Austria draw with Israel on the same night.
Clarke's team are aiming to reach back-to-back major tournaments - and a first World Cup since 1998.