Southampton's Fox and Wallace of Rangers - both left-backs - have been struggling with injuries and are the fifth and sixth players to withdraw from an initial 28-man squad.
Gordon Strachan had already called up Stevie Hammell as cover, and the Motherwell man is now in line to win his second cap in Zagreb.
Scott Brown and Gary Caldwell are among the other absentees, leaving Strachan with a decision to make over who captains the side, with James Morrison, Shaun Maloney and Allan McGregor the leading candidates.
Meanwhile, Scotland coach Scotland coach Stuart McCall issued positive fitness bulletins over defensive duo Grant Hanley and Gordon Greer.
McCall said: "Grant just rolled his ankle in training. It was just a precaution that he came off and got ice but he won't be a problem.
"Gordon Greer is coming up. He has had a little bit of tonsillitis but he will join up with the squad, as will Alan Hutton on Monday night."
McCall is confident Scotland will give a good account of themselves in Zagreb despite the lack of experience in the squad.
He added: "Everyone is looking fine and raring to go. We are just trying to formulate a system and a shape that will suit the players that we've got.
"We are under no illusions that Croatia is a difficult place to go. But we've got to go there with a game plan, where we can be difficult to play against."
The Scots have just two points from six Group A games and need two more from their remaining four matches to guarantee fourth-seeded status for the Euro 2016 qualifying tournament.