Matt Phillips and Phil Bardsley have been forced to withdraw from the Scotland squad for this week's friendly against Slovenia.
Blackpool winger Phillips is currently suffering with a hamstring injury, while Sunderland defender Bardsley has a calf problem.
The duo have joined Celtic captain Scott Brown, who pulled out of the squad over the weekend.
Scotland's assistant manager has revealed that it is unlikely that cover will be brought in for the injured trio.
"Phil was here last night," Houston said. "Matt Phillips is still here but they're not going to make it.
"At this moment in time nobody has been brought into the squad."
Houston has also confirmed that all of the players that participated in yesterday's Carling Cup final will take part in training this afternoon.
