Hearts left-back Wallace and Burnley striker Iwelumo are major doubts for Scotland's Euro 2012 qualifier against Lithuania in Kaunas on Friday night.
Wallace pulled up with an ankle injury in the warm-up before his side's 1-1 Scottish Premier League game against Dundee United at Tynecastle on Sunday and had to be replaced.
Iwelumo tweaked a hamstring near the end of his club's defeat against Swansea.
Turf Moor boss Brian Laws said on Burnley's official website: "We will have to assess what Chris is like, but I would say it is highly unlikely now he will go with Scotland during the international break."
Rangers boss Walter Smith said on Friday Andy Webster was out of the squad.
The Ibrox defender has sciatica but, like Wallace and Iwelumo, he will be assessed by the Scotland medical team when they meet up at their Mar Hall hotel base, near Glasgow.
In the first double-header of their qualifying campaign, Scotland play Lithuania on Friday and then Liechtenstein at Hampden on Tuesday week.
Steven Fletcher, Alan Hutton and Kevin McNaughton may only be available for the Liechtenstein game depending on fitness
Scotland boss Craig Levein has already lost number one goalkeeper Craig Gordon to a broken arm and defender Gary Caldwell, who had two hip operations during the summer.