It takes the total numbers of calls-offs to six since Alex McLeish named his squad at the start of last week.
The Scotland manager will look to replace both of the latest two injured players.
Armstrong is thought to have picked up an injury after coming on as a second-half substitute on Saturday as Celtic clinched the double treble with a 2-0 victory over Motherwell in the Scottish Cup final.
Souttar has not played since Hearts’ penultimate match of the season against Celtic. The centre-half is understood carrying a hip injury.
Scotland play Peru in Lima on 29 May before flying on to Mexico City to face Mexico on 3 June. McLeish had already been dealt blows with the withdrawals of Matt Ritchie, Barry Douglas, Ryan Fraser and goalkeeper Allan McGregor.
Motherwell’s Chris Cadden, Kilmarnock’s Stephen O’Donnell and Celtic goalkeeper Scott Bain have all been added to the travelling party in the last few days.