The 19-year-old arrived at the Scotland camp's Aberdeen hotel on Monday after being promoted from the under-21 squad.
Saunders joins Paul Caddis, Murray Davidson and James McArthur in being added to the pool.
Scotland manager Craig Levein has been hit with nine withdrawals from his original squad, which was already missing established stars such as Kenny Miller and Allan McGregor. A further five players were forced to decline late call-ups through injury.
Six players were called up on Sunday including Kilmarnock midfielder Craig Bryson and Blackpool left-back Stephen Crainey, whose sixth and most recent cap came in a 1-0 friendly defeat in Denmark in April 2004.
There were also call-ups for Wigan defender Steven Caldwell, Dundee United defender Garry Kenneth, Middlesbrough midfielder Barry Robson and Celtic's Shaun Maloney.
The nine who withdrew included Wolves defender Christophe Berra, Blackpool goalkeeper Matt Gilks, Wigan defender Gary Caldwell and Cardiff winger Chris Burke, who all suffered injuries on Saturday.
James Morrison pulled out with a knee problem after lasting 73 minutes of West Brom's defeat at Wigan, while team-mate Graham Dorrans was sidelined by a foot problem for that game and also withdrew.
Tottenham right-back Alan Hutton will have an injection on a persistent groin injury this week while Rangers pair Steven Naismith and Steven Whittaker both called off.
Levein tried and failed to bring in Rangers defender Kirk Broadfoot, Watford midfielder Don Cowie, Middlesbrough midfielder Kevin Thomson, Celtic full-back Mark Wilson and Cardiff defender Kevin McNaughton.
Even one of his late call-ups is a doubt with Davidson struggling with an ankle knock, but Levein insists Tuesday night's Pittodrie friendly (kick-off 8pm) is a worthwhile exercise.
"I can say 100% that this game will provide me with a lot of information," he said.
"Let's not forget some of the best players that ever played international football got their chance in friendly matches.
"If we get through this game and I have discovered another two players then I will be delighted with that."
Saunders' call-up is a surprise given his Motherwell team-mate Mark Reynolds was overlooked but the versatile defender was available given he had joined up with Billy Stark's under-21 squad ahead of Wednesday's friendly against Northern Ireland.
Swindon full-back Caddis was Scotland Under-21 captain during their run to a European Championship play-off defeat by Iceland while St Johnstone midfielder Davidson and Wigan's McArthur have both had international experience at the same level.