Glasgow Warriors centre Alex Dunbar requires ongoing assessment and treatment on a mild posterior thigh strain with Scotland's Six Nations opener against England 12 days away.
Dunbar sustained the injury as Warriors beat Racing 92 on Saturday.
Prop Alex Allan and flanker Hamish Watson have also been added to the Six Nations training squad.
Centre Mark Bennett continues to make good progress on the shoulder injury he sustained earlier this month.
Glasgow Warriors' Allan and Edinburgh's Watson joined the rest of the squad at their training base in St Andrews on Sunday.
Allan, 24, has a solitary Test cap to his name, having made his international debut against the USA in the summer of 2014.
Watson, also 24, has made two Scotland appearances, both against Italy, in the 2015 Six Nations and the Rugby World Cup warm-up fixture in Turin respectively.
Both players are products of the Scottish Exiles development programme.
Scotland's Six Nations campaign begins with the Calcutta Cup clash at Murrayfield on Saturday, 6 February.