Norwich City's Steven Whittaker has been called up as cover as Gordon Strachan had only 10 players taking part in training on Monday morning.
Celtic boss Neil Lennon revealed at the weekend that Forrest would be out with a back injury but no reason has been given yet for the withdrawals of McCormack and Fox.
The whole squad met up on Sunday night, but many have been given extra rest, including Sunderland's Steven Fletcher and other new squad members, Brighton's Liam Bridcutt and Dundee United's Gary Mackay-Steven, who played for their clubs on Sunday.
George Boyd has had his first call-up for the two games after scoring three goals in five games for Hull since moving from Peterborough.
Wigan's Gary Caldwell has not played since the beginning of the month and was an unused substitute in Sunday's win over Newcastle, but he did train with the squad at Marr Hall on the banks of the river Clyde.