Matt Phillips and Phil Bardsley will not travel with Scotland for the friendly against Slovenia after being ruled unfit.
Blackpool winger Phillips has a hamstring injury while Sunderland full-back Bardsley has a suspected tear in his calf. The pair join Celtic midfielder Scott Brown in pulling out of the squad. Wigan midfielder James McArthur was added after Brown withdrew but no-one else has been called up.
All three withdrawn players turned up at Scotland's Mar Hall hotel base in Renfrewshire for medical assessment.
"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.''
Debutant call-up Phillips missed Blackpool's 3-1 win over Bristol City on Saturday, while Bardsley played for Sunderland in a 4-0 defeat by West Brom but suffered the injury.
The England Under-20 international, who has lodged his change of allegiance with FIFA, was taking part in a photo shoot for Scotland's new away kit this morning and will return to Blackpool before the rest of the squad fly to Slovenia on Tuesday.
Brown strained his groin during Celtic's victory over Dunfermline last Wednesday and missed the weekend defeat of Motherwell.
Meanwhile, all three of the players who played in the Carling Cup final have confirmed they will join up with the squad.
Cardiff pair Don Cowie and Kenny Miller were on the losing side while Liverpool midfielder Charlie Adam claimed a winner's medal despite missing a penalty in the 3-2 shootout victory, which followed a 2-2 draw.
Houston revealed manager Craig Levein had spoken to Cardiff manager Malky Mackay after the game.
"We spoke to big Malky last night and the guys are keen to come,'' Houston said. "Kenny Miller, at the age he is at (32), is still keen to come. They might not make this afternoon's training session but they will do a recovery session with the physios.''