Huddersfield Town's Paul Dixon and Gary MacKenzie of MK Dons have been drafted into the Scotland squad ahead of the opening World Cup qualifiers against Serbia and Macedonia.
Southampton left-back Danny Fox (groin) and Norwich defender Russell Martin (ankle) have been ruled out of the first game against the Serbs at Hampden on Saturday.
Former Dundee and Dundee United left-back Dixon, 25, has been capped at Scotland Under-21 level while former Rangers and Dundee defender MacKenzie, 26, has yet to win his first international honour, although both players have been in senior squads in the past.
One left-back Scotland manager Craig Levein may have turned to, Lee Wallace of Rangers, has a hamstring injury.
Celtic defender Charlie Mulgrew reported to Scotland's Mar Hall base with an Achilles injury while Wolves centre-back Christophe Berra was excused from turning up on Monday as he awaits the birth of his first child.
Besiktas goalkeeper Allan McGregor is confident of being fit after recovering from the groin strain that forced him off in the friendly win over Australia last month.
"He played on Saturday and kept a clean sheet,'' Scotland assistant manager Peter Houston said.
"We are absolutely delighted he's fully fit because his experience in playing big international matches is vital.
"We were convinced after the Australia game that he was struggling. He had a couple of injections and recovered brilliantly. Fair play to (his club) for getting it sorted."