Bristol City full-back Richard Foster has been called into the Scotland squad after Charlie Mulgrew became the latest defender to pull out.
Mulgrew has succumbed to the Achilles injury that kept him out of Celtic's draw against Hibernian on Saturday and will miss the opening World Cup qualifier against Serbia.
Former Aberdeen captain and Rangers player Foster, 27, is uncapped.
The versatile Mulgrew joins a number of full-backs unavailable to manager Craig Levein.
Steven Whittaker and Phil Bardsley were missing from the original squad because of ankle injuries and Danny Fox and Russell Martin pulled out.
Levein called up Huddersfield left-back Paul Dixon and MK Dons defender Gary MacKenzie on Monday.
Wolves centre-back Christophe Berra was due to join up with the squad on Thursday after his partner gave birth to a baby girl on Wednesday.