The winger missed Bournemouth's 1-1 draw with Burnley at the weekend with a knee injury and he will not feature for the young Scots on Thursday either.
Coach Billy Stark has drafted Kilmarnock youngster Chris Johnston into the group to fill the gap.
Stark said: "Unfortunately we have lost Ryan again, which has been something that has happened fairly consistently.
"It's just one of those things. He was with us the last time and tweaked his hamstring and we had to send him back.
"This time it's a knee injury and it's just unfortunate.
"We've brought in Chris Johnston from Kilmarnock to fill that winger's slot."
Stark is also sweating over the fitness of several other players ahead of the clash in Paisley.
He added: "We have one or two wee issues with players, which is not that unusual.
"Jordan McGhee has a tender ankle and Kevin McHattie had a knock as well that we will have to go carefully with.
"Ryan Gauld is still feeling the effects of the knock he got on Saturday and Stuart Armstrong has a strain above his knee.
"We've got to watch them which isn't ideal because we're lots of bodies down training-wise."
Meanwhile, Danny Handling is hoping to make his debut for the under-21s this week after injury marred his first call-up to the squad last time.
The Hibernian midfielder said: "Hopefully I get a chance but I'll need to wait and see how it goes in training.
"The last time was a bitter disappointment because it was my first time getting called up to the under-21s.
"To be sent back through an injury that was a tackle over nothing really was pretty poor. The family were gutted for me as well.
"But I'm back fit now and I've been training and involved in games for a couple of weeks.
"It's nice to get another chance. I've got a chance to rectify things and hopefully I get that chance on the pitch.
"I feel back to my normal self so I'll just have to wait and see how it goes."
Scotland suffered defeat away to Georgia in their last outing and Handling says they are keen to make amends on home soil.
He added: "It's a game we are all looking forward to and hopefully we can get a good result and climb the table.
"If we get the right result it could enhance our chances of getting through the qualifiers.
"Hopefully we can get the right result on the night and a good performance."